[daanodinot]

Well there is, but it's a bit of work...

Your DVD ISO files contain multiple episodes in one iso file. What you can do is open the ISO file in a program like DVD Decrypter. That program can then copy the individual episodes to a folder of choice without compressing them or loss of quality.
Then you can add individual episode thumbs...

The thumbs are linked to physical files on your harddisk. Not to episodes...

Yeah.. unfortunately it requires episode ripping, while I'd like to retain the original dvd structure. BTW, the images that are scraped from tvdb are stored in the cache folder right? Of course, in that case, I could consider replacing those but that would be far from ideal. Maybe there is a way to do something with fake files, change something in the xml. I dunno... I find it hard to believe it's absolutely impossible to add individual thumbs when using iso files with multiple episodes.