The only way I know to do it is if the pic is on the web somewhere (like in a photo album or something). All the language for the sig, avatar, etc is the same as the post (hence I can change text color in mine)
To add an image on the net somewhere, do
{IMG}pasteaddresshere{/IMG}
but instead of the squiggly brackets, use the regular ones below them on the key (if I used the real ones, my post would turn into an image)