Glenn,
You didn’t mention what operating system you have. I’m going to assume Windows as I don’t know about the MAC.
When you save over an existing photo, it doesn’t overwrite the exact same area on the disk. Typically, the new file is written, the old file is marked deleted and the space it used is marked free, and the new file is given the old files name. The old space is not overwritten. If you have an "undelete" utility, and you act fast, you can generally recover the file. But, time is your enemy. The more you use the computer, the stronger the likelyhood that the original space will be allocated and overwritten.
Here’s an article that discusses recovery and lists some tools. It’s a very long url, you may have to cut and paste this into your browser if the full link isn’t recognized by your reader.
http://www.apcmag.com/apc/apcmag.nsf/EA67FCD4C82B07A1CA256AC 200040A09/4D81DA 6CAF1A8066CA256D1A00252067
The article is written about Windows 2000 but it equally applies to Windows 98, NT, ME, and XP. (Windows XP doesn’t support ntfs file system so that won’t apply to it of course.)
It’s probably too late to recover this file but you may wish to obtain and install an undelete utility for the next time.
If you have a Mac, someone else will need to tell you about whether undelete is available or possible.
Good luck,
Bob