Images in ebooks can be surprisingly large. Here’s how to optimize them without losing quality.
Why Image Optimization Matters
| Issue | Impact |
|---|
| Large file size | Slow page turns, uses storage |
| Wrong resolution | Blurry or pixelated images |
| Wrong format | E-reader may not display them |
| Too many colors | Wastes space on grayscale e-readers |
Recommended Image Specs
| E-Reader Type | Max Width | Best Format | Color Depth |
|---|
| Kindle Paperwhite | 1072px | JPEG / PNG-8 | Grayscale |
| Kindle Scribe | 1404px | JPEG / PNG-8 | Grayscale |
| Kobo Libra Colour | 1264px | JPEG | 16-bit color |
| Kobo Clara BW | 1072px | JPEG / PNG-8 | Grayscale |
| BOOX Tab Ultra | 1404px | PNG / JPEG | Full color |
| PocketBook Era | 1072px | JPEG | Grayscale |
| Format | Use Case | File Size |
|---|
| JPEG | Photos, gradients | Small |
| PNG-8 | Diagrams, screenshots | Medium |
| PNG-24 | High-quality images | Large |
| GIF | Simple animations | Small |
| WebP | Modern format (limited support) | Very small |
How to Optimize (Using Calibre)
- Open Calibre → Preferences → Conversion → Common Options
- In the Images tab:
- Set Max Image Width to 1072 (or as appropriate)
- Disable “Use Lossless Compression” for smaller files
- Enable “Remove margins from images”
- Apply when converting books
For JPEG Images
- Reduce quality to 80% — indistinguishable on e-ink
- Resize to 1072px wide (Paperwhite) or 1404px (Scribe)
- Convert to grayscale for Kindle/Kobo
For PNG Images
- Use PNG-8 (256 colors) instead of PNG-24
- Resize to max width for your device
- Run through pngcrush or tinypng.com
Batch Processing
Using ImageMagick (Free, Command Line)
# Resize all images to 1072px wide
mogrify -resize 1072x *.jpg
# Convert to grayscale
mogrify -colorspace Gray *.jpg
# Reduce quality to 80%
mogrify -quality 80 *.jpg
Using Calibre Plugin
Install the Quality Check plugin for Calibre to find oversized images.
How Much Space You Save
| Original | Optimized | Savings |
|---|
| 5 MB (full-color PNG) | 400 KB (grayscale JPEG) | 92% |
| 2 MB (large JPEG) | 350 KB (resized JPEG) | 82% |
| 500 KB (PNG-24) | 120 KB (PNG-8) | 76% |
Inksky