When an ebook needs an image
Visuals add file weight, production work, rights questions, accessibility decisions, and new ways for an export to fail. They earn that cost when they reduce cognitive effort or communicate something prose cannot express as efficiently. A decorative image can still be intentional, but it should not interrupt reading, carry hidden information, or receive a noisy description that creates no benefit.
Write the image brief before making the image
Complete this sentence: 'After this visual, the reader can see/compare/do ______ more easily.' If the blank only says 'the page looks better,' decide whether the visual is genuinely decorative and treat it accordingly.
How large should an ebook image be?
Start with its display size and the detail a reader needs. A useful illustration can be relatively small; a screenshot full of tiny labels may remain unreadable even at high resolution. Crop to the relevant control, simplify labels, or split the screenshot into two steps before increasing the file size.
For a print workflow that asks for 300 pixels per inch, an image printed four inches wide needs 4 × 300 = 1,200 pixels across. That calculation is a print example, not a universal ebook requirement. Reflowable ebooks must also be checked on screens; a “300 DPI” label alone tells you little about small-screen legibility.
Check one image before processing the whole set
- Keep the original and make a separate export copy.
- Try JPEG for a photographic scene and PNG when crisp edges or transparency matter, subject to your exporter and destination requirements.
- Compare the result at its intended size. Look at labels, thin lines, faces, and gradients rather than the file size alone.
- Open it in the exported ebook. If readers must zoom to understand it, change the composition or supply the information in nearby text.
Use the storybook sample files to inspect an actual illustrated export. Compare the PDF and EPUB versions of the same scene; the reading behavior is more informative than a gallery thumbnail.
Ebook image types and accessibility requirements
| Image role | What the reader needs | Appropriate treatment | Common failure |
|---|---|---|---|
| Informative | The meaning or observation conveyed by the image. | A concise alternative that communicates the relevant information; use a caption for context useful to everyone. | Describing colors and objects while omitting the point. |
| Functional | The action or destination associated with the image. | Name the function, such as 'Download the worksheet,' rather than the icon's shape. | Alt text such as 'arrow icon' that never explains what happens. |
| Complex | The main takeaway plus access to the underlying relationships or data. | A brief alternative and a nearby extended explanation, data table, or equivalent account. | Trying to compress an entire chart into one exhausting text attribute. |
| Decorative | No information; it should not interrupt the reading flow. | Mark it so assistive technology can ignore it; in HTML-based contexts this commonly means an empty alt value. | Announcing 'decorative flourish' on every chapter page. |
| Text in an image | The words and their purpose. | Prefer real text. When an image of text is essential, provide an equivalent and preserve legibility. | Embedding small paragraphs that cannot resize or reflow. |
The W3C Web Accessibility Initiative image tutorials organize image guidance around purpose and context. The same asset can need different alternative text in two chapters because the surrounding argument changes what matters.
Write alt text from the information the image contributes
Decide what someone would lose if the image were unavailable. Describe that contribution in context. The same illustration can need a different alternative in another chapter, and an image that only adds decoration should not be announced as another piece of content.
A screenshot that teaches an action
Unhelpful: “Screenshot of the application.”
Useful: “Export menu with EPUB selected.”
The nearby instructions should still say what to select and what happens next. Do not make the reader infer the whole task from the screenshot.
A chart with three values
For an invented bar-chart example, the data might be Monday: 8 sign-ins, Tuesday: 12, Wednesday: 9. A short alternative could say “Tuesday has the most sign-ins across the three recorded sessions.” Put the values in an adjacent table if readers need them. Do not say that a new welcome email caused the increase unless your evidence supports that conclusion.
A purely decorative chapter divider
If a small leaf illustration adds no information, use the export tool's decorative-image treatment where available. In HTML this is commonly an empty alt value. A visible caption that repeats “decorative leaf” is not a substitute for that treatment.
Captions serve all readers and can identify a source, date or context. Alternative text provides the relevant visual information when the image is not perceived. Check what survives in the exported file: writing an alternative in a planning document does not guarantee that an exporter inserts it into the ebook. See the W3C image tutorials for the purpose-based distinction.
Build an image brief the illustrator or generator can actually follow
Choose one passage that needs a visual and write the brief against its reader task. This example is for a welcome guide that teaches visitors where to go.
Example brief: arrival diagram
Reader task: find the garden-room entrance from the main gate.
Source: a checked floor plan and a recent site walk, supplied by the author.
Must show: gate, path, garden room and the accessible entrance, if verified.
Must omit: staff-only rooms, private names and decorative routes that do not exist.
Labels: keep them as editable text where possible.
Acceptance check: ask a newcomer to trace the route, then compare it with the actual site.
A generative image model can make a plausible building plan with a nonexistent door. For a navigational diagram, verified geometry matters more than visual polish. Build from the real plan and check every route; do not use a decorative generated map as evidence that an entrance is accessible.
When AI illustration is a reasonable fit
For a conceptual chapter opener, a brief can be much looser: “An overhead illustration of a welcome desk with blank sign-in sheets, a pencil and a folded map; warm paper texture; no readable text, identifiable people or logos.” It establishes a mood without claiming to document a real location. Label a conceptual illustration when readers could otherwise mistake it for a real example.
Keep production notes with the asset
Record the filename, source or generation tool, date, permitted use, edits, caption, alternative text and chapter placement. Keep the full-size original and create a delivery copy. If you later change the route or claim the image supports, revisit the brief and caption as well as the pixels.
Make screenshots and diagrams readable at ebook size
A screenshot can contain the right information and still be unusable after it shrinks. Suppose a 1,500-pixel-wide screenshot is displayed 300 pixels wide. Every element is reduced to one fifth of its original width; a 20-pixel label becomes about 4 display pixels high. More compression tuning cannot make that label readable.
Repair the composition before increasing resolution
- Crop to the relevant control. If the lesson is choosing EPUB in an export menu, the whole desktop is unnecessary.
- Split a sequence into separate steps. A screen before opening the menu and a close view of the selected option often explain more than one dense annotated screenshot.
- Move instructions into live text. Put “Choose EPUB, then download the file” in the manuscript. Numbered callouts can connect the text to the image, but should not be its only instructions.
- Test at the delivery size. Open the exported file at a normal reading size on the target device. If pinch-zoom is necessary for a basic instruction, simplify the visual or provide a separate enlarged reference.
For charts, retain the data
Keep axis labels, units and the data source with the chart. Describe what the comparison supports without adding a causal explanation the data cannot establish. When exact values matter, include a real data table or a linked accessible equivalent, rather than only a picture of the table. For a diagram, provide the sequence or relationship in nearby prose.
Amazon's reflowable-image guidelines address legibility in Kindle reading editions. These are distinct from the requirements for image-intensive fixed-layout children's books.
Image quality checklist for EPUB and PDF
- Every image has a recorded purpose and is placed next to the passage that uses it.
- Rights, licenses, model or property releases where applicable, and required credits are documented.
- Text inside images remains legible at realistic display sizes, or has been replaced by actual text.
- Information is not distinguished by color alone.
- Informative and functional images have contextual alternatives; decorative images are treated as decorative.
- Complex diagrams and charts have a nearby explanation or data equivalent.
- Captions identify sources or significance without needlessly duplicating the alternative text.
- Aspect ratios, crop, orientation, and reading order survive export.
- The edition remains understandable when an image is unavailable.
- File size and visual quality are balanced by inspecting the actual delivery file rather than trusting an export preset.
Image rights and Amazon KDP requirements
Image quality is not only a design issue. Before publishing through Amazon KDP, review the current KDP Content Guidelines and confirm you have the necessary rights for the material you submit. If KDP will convert the manuscript, also check its current supported ebook manuscript formats and inspect what the conversion does to image placement and readability.
- W3C WAI: Images Tutorial: contextual guidance for informative, decorative, functional, complex, and text images.
- W3C: EPUB 3.3: technical context for EPUB publications and their resources.
- Amazon KDP Content Guidelines: current content and publishing requirements for that platform.
Choose the format for your illustrated ebook
Reflowable and fixed-page editions place different constraints on captions, diagrams, tables, and exercises. Compare them in the ebook format guide, place the visual pass inside the complete ebook workflow, or explore the dedicated AI ebook generator with images page.