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KB0051: Why are some of my images fuzzy or low quality?

When Jutoh creates an ebook, the default behaviour is to convert all images that are not already in JPEG format (such as PNG or GIF), to JPEG. This results in a smaller book size, but can reduce the perceived quality and cause blurriness or artifacts, especially if the images are line art rather than photographic.

There are configuration properties that you can set that may help:

Set Convert images to JPEG to All except GIF, and replace your line art with images inserted from GIF files;
Set Image quality to a higher value, such as 90%. The default is 80%, which is good enough for most purposes.

You can also try inserting larger (higher resolution) files, and specifying the actual size (or maximum size) in the image properties. The file will be dynamically sized by the reader - but see the comment below about Adobe Digital Editions and large images.

Note that for Kindle files, Kindlegen will always recompress JPEG files. Amazon says that GIF files under 128KB in size will be left uncompressed, but sometimes this appears not to be the case.

Given that Jutoh already converts images to JPEG, you will achieve a higher quality if you insert the files into Jutoh as PNG, and set Convert images to JPEG to None in your Kindle configuration so that the conversion to JPEG only happens once.

Large and scaled images, and Adobe Digital Editions

You may find that large images (say, more than 1000px wide) still render poorly in Adobe Digital Editions and readers built on the same software, such as Bluefire Reader, giving a pixellated appearance. In fact ADE is poor at any type of scaling. The same image may render perfectly well on other platforms such as Kindle and Kobo.

It appears that Adobe's resizing code, at least on PCs, is to blame. You can either live with this and assume that most people will read the ebook on a device with better rendering, or you can replace the image with a smaller version, say 400px wide. However, if the image ends up being scaled, which will happen if you specify a percentage width (recommended), then the scaled image will still render poorly on Adobe-based readers.

You can limit all images in your book to a maximum size that you specify in your configuration using the Maximum image width option. This will physically resize the images on compilation. This way, you can shrink the images for some distributors and not others, by using different options for different configurations.

Note that we have confirmed that the pixellation occurs for scaled images even if the code generated by Jutoh is simplified to the most basic possible HTML, so we have eliminated the possibility that mixing CSS and inline styles, or wrapping the image in an extra div to work around an Apple Books bug, is to blame.