A few things you could try... (good luck!)
Reset the Messaging App
Settings>Menu>Applications>Manage applications>Soft menu key>Filter>All. Scroll to the Messaging App and tap that. Tap Clear Data, Clear Defaults, then Force Stop. Powercycle the phone. Test MMS. If it fails to resolve the issue go the messaging application and tap the Menu soft key>Settings>Menu soft key again> Restore Default Settings.
Alternative Messaging Apps
Try ChompSMS or Handcent from the Market.
Download the message first
I forget how you do this exactly :o/ Maybe long press or menu button? Then try viewing the image from the Gallery app.