Unless you have a NFO file with your patch, specifying on wich image it should be applied, I doubt you can guess what it originaly was. Sometimes, there's some text in PPF files. You can read it with some tools here :
http://www.megagames.com/dc/dc_ppf.shtml
Otherwise, if you've got a non boot game, try making an image with CDRWin. If it's a selfboot game, use DJ.
See what's happening when you patch it.