I have the exact same problem. I was editing files for a friend on photoshop cs3 and was overwriting the original files with the edited versions. Eventually I just ended up saving as a jpg instead of clicking save. I can't open any of the pictures I changed to jpg in windows picture viewer, live picture viewer, paint, cs3, or any other viewer. It only works on quicktime picture viewer. Sadly I can't change the file type since it claims it is a jpg. My suggestion would to just instead of trying to save it as a jpg, just click save and overwrite the file. If you want to gain any of the pictures that are corrupted back, I found that opening them on quicktime picture viewer, clicking copy, and pasting them onto a new file on photoshop seems to work well. I don't know why they won't open, but at least you can get your pictures back.