I'm just wondering if it's possible for os x's e-mail program to show background images? Sometimes i'll recieve an e-mail from a friend, and it will give me the image that they intended to be used for the background as an attached image....?
In the Viewing tab of the preferences, do you have 'Display images and embedded objects in HTML messages' ticked? I'm not sure if this will fix it (I've got no messages with background images to test it on), but it's worth a shot.