To make this topic more confusing I'd like to say some stuff here.

1st off all:

SpiderTech: What is the file extension of the image?
This simple thing could just solve it all. If it was PNG though, Windows should still be able to open it up, and I think it is even able inside off outlook.

Personnally, When I try to save a file from Paint, It wants to save as .PNG , While I thought it's JPEG by default.

Furthermore, I think it's highly unlikely that that this is a mac/Windows problem, because they don't contact. The Mac just sends an e-mail so some destination, what happens to be Exchange (right? or is it a web client?) and it just saves there.

I do not know what mail client the mac is using, but if That's outlook 2008 (not really sure at this point, cos I'm a Windows user) There shouldn't be a problem at all.
Same with web clients like Hotmail and Gmail. That isn't related to OS at ALL.

This leaves me with the only cause as, File size limit exceeded, or unsupported File
extension by either Outlook, or Windows.

Too bad I don't know anything about Mac.

Hope I was Usefull, or else extra confusing