I figured it out. I just upgraded the account I have with my webhost, and in doing so my site was moved onto a different server. Their nameservers were correctly routing my domain to the new server after a few hours, but then the next day there was a period when it shifted traffic back to the old server (which should have been deleted). It was during that short but fateful period that I ftp'd that pic, and as soon as the nameservers shifted stuff back to the new server (where that pic didn't exist) it disappeared. I can't edit forum posts, but I just realized that all I have to do is to upload the pic again and make sure it's named the same. So hold on....
There. Check that shizzle out.
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100 pounds of shit in a 25 pound sack.