"I tried your code and it worked! But only once. When I ran the script again it didn't flip the picture back. ?!"

It's possible the image is being cached: what's displayed in the browser may just be a copy of what was displayed from the last run. Check the actual file on disk to see if it's being flipped between runs.

If it is a caching issue, you may need to set a short expiration date. See CGI: CREATING A STANDARD HTTP HEADER for details.