Yeah, I checked, and the checkbox for thumbnails was set. When I opened gthumb, all I saw was a bunch of placeholder images instead of the expected thumbnails. I already deleted that directory before, as well as ~/.gconf/app/gthumb and ~/.thumbnails. No effect. Is that weird, or what??
However, I have good news. On the browser preference page, I saw the checkbox 'Determine image type from content'. I checked it, and now the thumbnails appear. I unchecked it again, and the thumbnails are STILL there (I knew that I didn't have it checked before). So the problem is solved. Somehow.