[mstevens]

There's also the question not of how to do this but rather of whether you should do it.

Signatures are among some of the most annoying things on earth to some people to begin with. Images in signatures would just up the ante.

If I had to pay for data transfer (as some people do) and some eejit made me pay to download some picture every time I got an email from him, I'd do whatever it took to ensure I never got another email from him.

Maybe that's your goal, but if not you should be aware of the possibility.