Jury's out. I haven't had to deal with image files in signatures before, so it's a bit of a question mark for me, still. We'll explore the policy as things go on.
For the time being, I would say, keep the height and width of any images you put in your signatures to an intelligent minimum -- not too much larger than the 'Jim-Butcher.Com Community' graphic you see at the top of the page, which is 400 pixels wide and 50 pixels tall.