I was wondering about this myself. I was a bit confused for a while because it wasn't exclusive to WowInterface; Curse's pictures were not loading properly either. After some thought, I figured it out: some of FireFox's plugins are not working as intended.
- FireFox 16.0
- GoPhotoIt 1.4 <<< disabled now
- schillr 1.7 <<< disabled now
With these two turned off, pictures load properly for me. What if you don't have those plugins? Check your other plugins, see if any affect pictures, thumbnails, etc, and turn them off.