What email app are you using to send the attachments? If Apple Mail, you have to have the "Send Window's friendly" checked in the "add attachments" window. If MS Entourage, you have to encode "for all computers-Apple binhex".
Also make sure that you have the Show File extensions (.jpg in this case) on any save boxes. Finally, check Finder preferences/Advanced and amke sure that "show all file extensions" is checked.
Using OSX 10.3.9 & 10.4.11 on a G4, G5 & Intel Macbook