You could probably get back part of your picture by trying to open it in a variety of programs... Try Preview, GraphicConverter, Safari, Internet Explorer, and any other JPEG-viewing apps you have. Sometimes the program will load part of the picture, then you can either Shift-Command-4 and take a picture of that part of the screen, or try re-saving it.
Usually, once a JPEG is corrupted, its gone forever.
Best of luck!