Alt text has a real semantic meaning, so I didn't want to do that, but I added a throbber background image to images without any alt text. I just added it now, so it may take a few hours before you'll be able to see it due to caching.

Does anyone notice any problems with this? In order to achieve the desired effect, I had to give all images with no alt text display:inline-block, which I think shouldn't affect layout at all, but I could be wrong.