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See also:
What PPI or DPI should I use in my images?
How can I size an image relative to the current text size?
Apple Books fails to size an image when the height is specified as a percentage
Why do images, including the cover, expand incorrectly on some readers?
Why are some GIF or JPEG images not displayed in my Epub using Adobe Digital Editions?
How can I reduce the file size of my book?
Working with pictures in the Jutoh manual
This is a tricky question. There is no one answer to this; it's a compromise of quality and file size. Consider a width of 500 to 1000 pixels for a full-width image, or a higher resolution if targeting a print book. Whatever the original pixel size the picture is, you can always set the displayed size in the image's properties to the same or a different amount, either a percentage (recommended) or absolute. So you can use 100% for the width if you want to stretch the image to the display width. For very tall images, you may need to use a percentage height (and no width) to avoid cropping; a value of 75% will allow space for some text above or below the image.
Don't forget to put each image in a paragraph with no margins or indent, given that a 100% display width would then cause the image to spill over the right margin if there is a non-zero left margin that pushes the image to one side. Or your image can be off-centre if there is an indent.
Don't specify both a percentage width and height in image properties, just one dimension, or your image could lose its original aspect ratio.
Also, don't specify just a maximum width or height without specifying an actual width or height, since Adobe Digital Editions and derived software take this as an invitation to scale the image up to the maximum display size.
Kindle ignores maximum width and height values, and Apple Books cannot handle a percentage height or maximum height.
If you use an originally large image, then the extra size will make the image sharper on a suitable hi-res screen. On the other hand, it may make the book file size unacceptable. You could always try using large images, and then if creates a file that's too big, physically limit them using the configuration option Maximum image width, using e.g. 400 to size them to at most 400 pixels wide. This option affects the actual image dimensions, and not just the displayed size. By using configurations in this way, you can have different physical image sizes in different configurations, for example smaller ones in Kindle than Epub.
Also look at the guidelines for the platform (e.g. Apple Books, Kindle) to see the recommended maximum sizes.
Very large image sizes (for example over 2000 pixels in width) can cause the reading experience to suffer if loading pages becomes slow. As usual, do test on as wide a range of software and devices as possible.
Your book may be rejected from Amazon KDP if your images are very large (e.g. 3000x4000). If you experience a book rejection without explanation, try reducing your image sizes.
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