finally had the idea to use the command line to look at the card instead of the Mac Finder...
In the .kobo/ directory there is a sqlite instance with 16M and an image/ dir with nearly 60MB. This contains, for each file on the sd card, three files, prefixed with "file____mnt_sd_" followed by author and title and then the suffix epub - N3_FULL.parsed, epub - N3_LIBRARY_FULL.parsed, and pub - N3_LIBRARY_GRID.parsed, respectively. I don't know whether these files contain the book covers as well, but with a size of 50-300K per file and the name of the directory it is at least probable.
Interestingly, I even have kepub directory with a few entries even though I haven't yet bought a Kobo book (and they aren't shown either).