Originally Posted by Koboyashi
The Kobo keeps cover images internally as GIF's. If you try to save your cover e.g. with Photoshop as GIF image, you'll see similar patches. Attached is a screenshot from the Photoshop dialog before saving. No idea how to avoid this - maybe it needs more dithering in the original cover.
And the grayscale rendering on the Kobo seems to increase the problem additionally.
Thank you for taking the trouble and for your reply.
I converted the jpg cover to gif - grayscale with the GIMP
Although it has some problems they're not nearly as pronounced as on the Kobo. Could it be that the Kobo software uses a very low resolution?
Anyway, thanks for the info.
I'd love to hear from people who have other epub devices.