Images are often used in eBooks. However, there is often a need to edit or manipulate the graphics for use in the eBook.
 The need
Images typically increase the size of eBooks tremendously, often doubling or tripling the size of the eBook. They can also slow down the reading experience. (Of course they may also increase the reading experience.) Some eBook readers will scale large images to the screen size but will fail if the image is too large (say greater that 4 Mpixel). Large size may be either file size or image dimensions, image size. File size affects the loading time while image display size affect the viewing display time.
The image size can be reduced a number of ways such as: reducing the overall file size, reducing the image dimensions, reducing the number of colors, and converting to grayscale. For JPG you can also reduce the image quality. Changing the format of the image might also reduce the file size without changing the display quality.
Some eBook Readers now permit viewing the image by itself with increased size and using scrolling. This feature may require a higher resolution image that would have been needed on eBooks without this feature.
The following free tools are available to manipulate graphics.
- IrfanView is an excellent graphic manipulation program. It has a batch mode built in to adjust image sizes or other qualities. It is only available for Windows. It is a free program.
- Imagemagick is a image manipulation program available on a number of platforms. Binaries are available on Unix, Mac OS X, iOS and Windows It also has an API available that allows it to be the engine behind many other programs.
- PhotoScape - free photo editing software and split and combine files, edit content, make batch changes, and more. It is only for Windows.
- Scan Tailor is an interactive post-processing tool for scanned pages. It performs operations such as page splitting, de-skewing, adding/removing borders, and others.
- xnconvert is a program available on multiple platforms specifically design to batch convert images. Binaries are available for Windows, Mac, and Linux.
- XnView is an efficient multimedia viewer, organizer and converter for Windows. It claims to support 500 formats. It is free for personal and education use. XnViewMP is the advanced version and is available for Windows, Linux, and Mac in both 32bit and 64bit versions.
- Inkscape - vector editing, exports several formats and can edit SVG.
 Tips on Graphics for eBooks
Here are some tips to get the best results for graphic images in eBooks.
- eBooks formats that support PalmOS devices will often have a 62K file size limit for one image. To keep the creation programs from altering your image you may want to adjust it yourself.
- Some eBook formats have alternate images for different screen sizes. Be sure you consider this when creating an eBook for distribution.
- Reducing the number of colors, number of layers, and the image size can greatly reduce the size of the image and thus reduce the size of the eBook.
- JPEG image size is greatly influenced by the quality setting. For eBook use you can often decrease this setting with no visible change in the image.
- Most dedicated eBook readers are monochrome while most other devices that can read eBooks are color. If your target device is monochrome you may want to convert the image to monochrome to decrease its size and perhaps improve the look However be careful.
- If the format is also used on PDA's you may want to keep it in color for those platforms.
- Reducing the bit depth too much can come back to haunt you. For example the original Sony e-Ink device supported only 4 gray levels. If you reduced the image to 4 gray levels the image will look worse that it could in the latest reader which can support 16 levels.
- When reducing colors avoid dithering. Dithering is a technique used to simulate higher color or gray image levels by mixing black and white dots spaced close together to simulate a gray. This is the technique used in newspapers to produce gray images. Used carefully it can work well but indiscriminate use can cause a speckled appearance to the image.
- consider clear borders around images to keep text from crowding the image. Some formats such as ePub permit adding a border around the image in CSS.
- Some readers are capable of zooming in an image. Don't make the original, such as a map, too small to be able to display the information. However, you may need to cut the image in half to make it viewable.
- Jpeg can blur simple graphics or add artifacts to the image. Use GIF or PNG for line drawings, particularly if the image is sparse.
- Images can show text. This can be a work around to display text when you don't have the correct font or text size available in the reader.
- Increasing the chroma level can often improve dark images better than any other technique.
- Know the pixel size limit of your target device. Most devices do not support a full screen mode so the graphic pixel size is less due to progress bars and other items that are always on the page. Making your images conform to the maximum will result in the best quality image since it will not have to be shrunk to fit.