The file I'm trying to open ends with jpg+.sit
A .sit file is a Mac archive (compressed) file. The only program which can read it is Stuffit. But if I read your post correctly and the attachment ends in jpg.sit then do not open it. Dual extensions almost always indicate a virus. Email your friend to verify that it did indeed come from him/her and if you still need to look at, save it to a diskette first and scan it.