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Here is a more detailed explaination of the problem, as I understand it:
Normally the first time the book cover is needed for any reason (display as a thumbnail, sleep screen, etc.) the firmware wil generate a set of images of various sizes and store them for later use. The next time they are needed they are loaded from the previously saved image. (This doesn't apply to the cover you see when reading the first page of a book, as that is loaded from the book itself each time, like any other picture in the book.)
For some reason the current firmware is saving very low resolution cover images for books on the external micro SD card. So when the image is needed for the sleep screen, it looks horrible.
Calibre can (if configured to) send pre-generated images for each book, which means that the device doesn't need to generate and save its own cover images, and so the very low resolution images do not get saved.
There are also some other advantages to using Calibre to send the cover images: The images will be displayed in thumbnail/home screen without any white bars You can choose to preserve aspect ratio or stretched to 3:4 ratio. You can send black-and-white images which save some space.
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