The OPDS feed has links to the thumbnails but the files are all named cover.jpg which compared to the nicely named author_booktitle.epub is less useful. (However the webpage for each book has a link to a thumbnail_*.jpg cover file, but this is not listed in the OPDS.)
In the spirit of saving SE hosting costs, in my private use I have been as careful as I can to avoid redownloading books and data, or preferring to make Microsoft (Github) pay for it by using github when possible.
I can take the bulk download list down if that is helpful to prevent any excess general bulk downloading, I just thought: "a reader asked for help, but was told 'you need to be a programmer,' here is XML (twice now), which didn't seem like a super helpful or friendly answer". However I don't want to drain your resources, Alex.
Jared.