To the best of my knowledge this not possible. If you are using the OutboundAppend method like I think you are there is no option there to use html tags (or other tags) to place images.
It may be possible to setup something like this in Outlook but if you plan on enforcing it over a large number of users then you're going to have a horrible time.
Most importantly of all though, this is simply a horrible idea. Signatures suck up enough bandwidth as it is without people sticking images in them too. Not all users have rich text or html mail so they'll just end up with a useless attachment for nothing. And another thing, even if you were going to do this, BMP is the worst possible format you could use.
My advice is don't even try it.