If you overwrite a file, you have changed the file contents, there is no backup data kept within the file itself.
BUT!! If this is a photo, and you did say it was, then where is the original? Where is your camera or the card that the photo was on before you put it on your computer?
I just uploaded my photos, and was going through editing some of them and I cropped a picture just to see what it'd look like, and I accidently saved over the whole file. Is there a way that I can retrieve it?! I've looked at restoring, but the only point I can restore from is about a week earlier when the photo wasn't on my computer. Any help?!