are the emails coming from people using outlook, or from the hotmail/outlook.com web mail?

Thanks for responding Matt, The graphics are not from people using Hotmail or Outlook. My email accounts are Hotmail and Outlook though.

I think Matt may be onto something here.

However, just on the side: Do you use AdBlock Plus in your install of Tb ? Also and this may be an altogether different troubleshooting direction, I imagine that the missing "embedded" images are referenced as remotely loaded files. Try to look at the source of your received emails and see whether the missing images are indeed referenced as remote. Tell us more.