Retrieve thumbnails from iPhoto?
According to this website, the thumbnails are stored inside ~/Pictures/iPhoto Library, which you can access by right-clicking iPhoto Library and choosing to view the package contents.

The first location to check is inside of the Thumbnails folder, as mentioned by OP below.

The second place to look would be in Thumb64Segment.data, or ThumbJPGSegment.data. Therefore, I would back up these files and see if there are any applications which can read this file format. I haven't been able to find one after a brief search on the Internet, but I'll update this answer if I do.

A third option might be that this may be happening because of a corrupt iPhoto Library. There's a tool called iPhoto Library Manager, which has a feature called First Aid that let's you:


  Rebuild or extract photos from corrupted iPhoto libraries


I haven't tried the software, so I don't know if it will work in your case, but they do have a trial version available.