Sorry, but at this time I am not inclined to allow that option.
On one of my other sites, I do allow graphics in the signature area, and it is a real eyesore to behold. I am constantly having to ask the member to NOT use a 1024x768 image there.
Eventually, as membership and traffic grows there, I am certain I will have to remove that option.
Since the signature area does not have any inherent limits on the size of a graphic (as does the avatar), I am certain I would have the same problem here.
Not to mention that it makes reading a thread rather difficult with all the distracting banners interlaced between nearly every post.
And there is the additional concern of bandwidth for those poor unfortunates still using dialup. It would add SUBSTANTIAL download time for every thread that contained posts with those embedded graphics.
Sorry, but from what I can see, there are far too many negatives to outweigh the positives.