So it looks like every once in awhile, they change the way images are stripped/optimized, and that means a whole new set of MD5s to worry about.
It'll be, you saved an image awhile back, drag it to the MD5 window to search for it, and it'll only show you old results, or sometimes no results at all.
The only way to get the new MD5 would be to post it and re-save it, which is stupid.
This really messes with image search, and there ought to be a way to deal with this.