Just looked again at the site:example.com and the number was back to what it was earlier today.
I realize you can hit different data centers. I haven't seen 4000+ of our images in the index since 2006 and there was a ton of newer photos in there. Can I assume that the 8x larger result I saw two hours ago is a new crawl that will be rolled out to the other data centers?
After reading your post, I did the site:example.com search in Images and page 1 reflected 608 images. I noticed a newer image in the result set. Clicking over to page 2, it now said "Results 19 - 36 of about 4,080". A ton of recent photos in the results.
As I paged through subsequent results, the total would change from 608 to 4080 every few pages, as would the amount of recent photos.