How to Write Alt Text: Easy Tips for Images in Digital Books
Digital books can contain photographs, illustrations, diagrams, charts, maps, tables, and other visual elements. But what happens when a reader cannot see those images?
Alt text, or alternative text, provides a text alternative for images so that the information or function they convey can be communicated to people who cannot see them. W3C guidance recommends choosing the text alternative based on the image’s purpose and context rather than simply describing everything visible in the image.
For publishers producing accessible EPUBs and other digital books, writing useful alt text is therefore an important part of content preparation.
The good news? You do not need to describe every visual detail. You need to communicate what matters to the reader.
What Is Alt Text?
Alt text is a text alternative that communicates the information or function of an image.
For example, imagine a digital textbook explaining photosynthesis. A photograph of a green plant might not need a detailed description of its leaves, background, lighting, and colors. If the important point is that the image shows a plant absorbing sunlight, the alt text should communicate that relevant information.
W3C recommends that alternative text describe the information or function represented by the image, with the exact wording determined by the context.
Why Does Alt Text Matter in Digital Books?
Readers who use screen readers or other assistive technologies may rely on text alternatives to understand visual content.
For digital books, this becomes particularly important when an image contains information that is not available elsewhere in the text.
For example, a historical photograph may illustrate an event discussed in a chapter. A scientific diagram may explain a process that the surrounding paragraph does not fully describe. A chart may contain data that cannot be understood from the chapter text alone.
The goal is not to describe an image for its own sake. The goal is to make its relevant information available in another form.
Start With the Purpose of the Image
Before writing anything, ask one simple question:
“Why is this image here?”
The answer usually tells you what the alt text needs to communicate.
An image might be:
- Informative — it conveys information.
- Decorative — it adds visual interest but does not contribute information.
- Functional — it acts as a link, button, or control.
- Complex — it contains substantial information that cannot be conveyed in a short description.
- An image of text — the text inside the image itself is important.
W3C’s image guidance uses these categories to help authors determine appropriate text alternatives.
How to Write Alt Text for Informative Images
For a simple informative image, keep the description focused on its meaning in the current context.
Example
Image: A photograph of a researcher examining a microscope.
Weak alt text:
“Person in a laboratory.”
More useful alt text:
“Researcher examining a sample through a microscope.”
The second version communicates the action that may be relevant to the surrounding content.
W3C recommends concise descriptions for informative images that convey the information or meaning represented by the image rather than simply listing visual details.
How to Handle Decorative Images
Not every image in a digital book needs a description.
If an image is purely decorative and does not contribute information, it should have an empty/null text alternative rather than unnecessary descriptive text. W3C gives alt=”” as the appropriate approach for decorative images in HTML.
For example, consider a decorative flourish used between two sections of a novel.
Adding:
“Decorative blue floral design”
may give the reader information that does not help them understand the book.
The better approach is to treat it as decorative when that reflects its actual purpose.
Writing Alt Text for Functional Images
Sometimes an image performs a function rather than simply displaying information.
For example, an image might be used as part of a link or button.
In these cases, the text alternative should communicate the function or destination, not simply describe what the image looks like.
For example:
Weak: “Arrow icon”
Better: “Go to next chapter”
The second description tells the reader what the control does.
What to Do With Charts and Diagrams
Charts, diagrams, maps, and infographics can be more challenging.
A short description may identify the image, but it may not communicate all the information contained within it.
W3C recommends providing the important information from complex images through an appropriate text equivalent, such as a longer description or accompanying content.
Example
Chart: A line graph showing annual sales from 2020 through 2025.
A short alt text might identify the chart and summarize its key message:
“Line chart showing annual sales increasing from 2020 to 2025.”
If the exact values or trends are important to understanding the chapter, those details should also be made available in text rather than relying only on the graphic. W3C provides examples of using accompanying text, tables, or longer descriptions for complex images.
How to Handle Images Containing Text
Some digital books contain screenshots, scanned pages, historical posters, advertisements, or diagrams with text inside the image.
If the text in the image is important and is not otherwise available as actual text, the alternative text should include the relevant text. W3C specifically recommends providing the same words when an image of text is used and the text is not otherwise available.
However, if the same text is already provided as real text nearby, repeating all of it in the alt text can be unnecessary.
Avoid Repeating the Surrounding Text
One of the easiest mistakes is writing alt text that simply repeats the paragraph immediately before or after an image.
Imagine a textbook says:
“The water cycle consists of evaporation, condensation, precipitation, and collection.”
The accompanying diagram illustrates those same four stages.
The alt text does not necessarily need to repeat the entire explanation. It could briefly identify the diagram and direct the reader to the relevant surrounding content, depending on how the information is presented.
W3C notes that an image may be considered redundant when its information is already sufficiently conveyed through nearby text.
Should You Use “Image of” in Alt Text?
Usually, there is no need to begin alt text with phrases such as “image of,” “picture of,” or “photo of.”
W3C and DAISY guidance both note that assistive technologies can identify an image, so adding generic phrases can make the alternative text less concise.
Instead of:
“Image of a historical building in London.”
consider:
“Historic building in London.”
The exact wording should still depend on the purpose of the image.
Alt Text Examples for Digital Books
| Image Type | Example Alt Text |
| Informative photograph | “A researcher examining a sample through a microscope.” |
| Historical photograph | “Workers operating machinery in a textile factory.” |
| Decorative illustration | Empty/null alternative when the illustration is purely decorative |
| Functional image | “Next chapter” |
| Simple diagram | “Diagram showing the four stages of the water cycle.” |
| Complex chart | “Line chart showing annual sales increasing from 2020 to 2025.” |
| Map | “Map showing the locations of the publisher’s three offices.” |
| Image containing important text | Include the relevant text when it is not otherwise available |
| Formula or scientific graphic | Describe the information needed to understand its role in the surrounding content |
These examples illustrate the principle rather than prescribing one fixed format. Context determines the appropriate alternative text.
Common Alt Text Mistakes to Avoid
1. Describing everything
Alt text is not necessarily a visual inventory.
You usually do not need to mention every color, object, background element, or visual detail. Focus on information relevant to the image’s purpose.
2. Using the file name
Names such as IMG_4837.jpg do not communicate the meaning of an image.
Write a meaningful text alternative instead.
3. Repeating nearby content
If the surrounding text already provides the same information, repeating it can make the reading experience unnecessarily long.
4. Treating every image as informative
Some images are genuinely decorative. Adding unnecessary descriptions can create additional content for assistive-technology users without adding useful information.
5. Ignoring complex images
A short label is not always enough for a chart, map, or detailed diagram. If the image contains important information, make that information available in an appropriate text format as well.
Alt Text and EPUB Accessibility
For publishers, alt text should be considered during the content-production workflow, not added as an afterthought.
The DAISY Consortium’s guidance for accessible EPUB production notes that non-decorative images should have descriptions, while decorative images can be marked as decorative. It also notes that an EPUB can still be generated when images lack descriptions, but the resulting book may not meet the needs of all users and can fail an accessibility check.
This is why image accessibility should ideally be addressed at the editorial or production stage.
A practical workflow might look like:
Source manuscript → Image review → Image purpose identified → Alt text written → Complex descriptions added where needed → EPUB production → Accessibility testing
The earlier accessibility information is captured, the less likely it is to become a last-minute remediation task.
A Practical Alt Text Checklist
Before an accessible digital book is delivered, review each meaningful image.
Ask:
- Does the image convey information?
- Is it decorative?
- Does it perform a function?
- Is the description relevant to the surrounding content?
- Does the alt text communicate the image’s purpose?
- Am I repeating information already available in the text?
- Does the image contain important text?
- Is this a complex chart, diagram, map, or infographic?
- If it is complex, is the important information available elsewhere in text?
- Have the final EPUB and its images been accessibility tested?
This approach is more reliable than applying a single word-count or character-count rule to every image because W3C guidance emphasizes purpose, context, and the amount of information an image conveys.
Frequently Asked Questions
1. What is alt text in a digital book?
Alt text is a text alternative that communicates the information or function of an image to readers who cannot see it or cannot access the visual content directly.
2. How long should alt text be?
There is no single length that works for every image. W3C recommends making alternative text as concise as possible while still communicating the image’s relevant information. When an image contains substantial information, a longer description or equivalent content may be needed.
3. Do decorative images need alt text?
For HTML images, purely decorative images should generally use a null/empty alternative such as alt=”” so assistive technologies can ignore them.
For EPUB workflows, publishers should follow the applicable EPUB accessibility requirements and production guidance.
4. Should alt text include the words “image of”?
Usually, no. W3C and DAISY guidance recommend avoiding unnecessary phrases such as “image of” because assistive technologies can identify images.
5. How should publishers write alt text for charts and diagrams?
Start with a short description that identifies the chart or diagram and its key message. If the image contains substantial information, provide the important details elsewhere in an accessible text format, such as a longer description or data table.
Conclusion
Good alt text is not about describing every pixel. It is about making the important information or function of an image available in text.
For digital-book publishers, the most useful approach is to evaluate each image in context: determine its purpose, write a concise alternative for informative images, leave genuinely decorative images without unnecessary descriptions, describe functional images by their purpose, and provide additional text for complex visuals when needed.
When alt text becomes part of the editorial and EPUB production workflow, accessibility becomes less of a last-minute task and more of a natural part of publishing.