Not using the default chat system, no. You'd have to completely script your own chat system in order to make changes to your liking, but I can imagine it being entirely possible to implement images in a custom chat system.
At first I wasn't quite sure why anyone would want to put images in chat, but I figure if it's not fully controlled by the player as to exactly what image is used, and when, then I could see something like a scripted chat system where emoticons like
and
are shown when their respective character combinations are entered.