Hi- you can only do this if you have access to the original photo. If you simply have a scan of the original, you are probably out-of-luck. What you need to do is place it on your scanner and set to scan at a fairly high resolution (300-600 dpi or greater). Then use the capabilities of your scanner to focus specifically on the area of interest, so that when you save it it doesn't save the whole thing, only the area you want.
Look at your scanner documentation if you don't know what its capabilities are.
Nick
