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Tanjamuse - Not in the Book Folder itself.
You can put a link to an image in Comments (or similar) but be aware it will display full size - click the last icon in the second group ==>>

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You can browse to an image on your computer or paste in a web URL and tick the 'this is an image' box
There maybe a way to resize the image to fit available width/height of the Book Details display area - but I have no idea what that would be. Note there are 2 book details display areas to consider - 1. the one on the left which would need to resize according to width, 2. the one on the bottom which would need to resize according to height. I would suggest the one in the free standing Book Details (behind the 'i' key) always display the image full size.
BR