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.
I am noticing a change from last few hours before the image loads and it looks good. Great job.
Update:
Ok, seems like it's not workingIt's working perfectly fine now. Confirmed feedback from my side.