If you get mail with a picture from somebody sending mail with Eudora and you try to read it in Mail.app you might need to tell them to re-send that picture as an attachment and not embedded between lines of text. If they don´t the only thing you see of that picture is a tiny little purple icon. Me and a friend triple checked this in mail sent with Eudora (5.x) and we also used AppleDouble, AppleSingle, BinHex and Uuencode data fork. So ask them not to paste or drag & drop any pictures between lines of text. They can paste the picture after the last word or as an attachment.
I don´t know if Mail.app is to blame but I know the problem will end up in Apple's Mail.app for MacOS X. The different encodings do not affect the jpeg-picture or the message.
URL of a bad example in Mail.app
Mac OS X Hints