PROBLEM SOLVED!
As it turns out, it was just a coincidence that I happened to get an unopenable file right after I did all that junk to my machine. PictureViewer opens other JPEG files on my machine without trouble, and other people were unable to open the file on their machines.
Good news: I didn't screw up anywhere - the file was the problem.
Bad news: I won't get to see the JPEG, which I have been promised is a voluptuous image of Liv Tyler!
Thanks to The Dude who helped me out over AIM!
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Fyre4ce
"I need a vacation." - Terminator robot