These are full colour png thumbnails used by Kobo in the I'm Reading / Books lists and as the Off Screen for the current book you're reading. The formats are as follows:
xxxxxxxxxxxxxxxxxxxxxx - bbMediumGridList.parsed - 93 x 135
xxxxxxxxxxxxxxxxxxxxxx - iPhoneThumbnail.parsed - 103 x 150
xxxxxxxxxxxxxxxxxxxxxx - NickelBookCover.parsed - 500 x 725
The xxxxxxxxxxxxxxxxxxxxxx identifiers are stored in the ImageId column of the Content table in the SQLite DB -
see this thread for more info