The concept would be this....
Similar to Live Journal. The user could specify X number of images or avatars to their profile. Specifying one as their "default" image. Then when the user posts their message they can choose from a popup the avatar they wish to associate with this post. A user could choose to have different images displaying depending on their mood.
I would venture to say this would be an "VALUE" added mod thus only available to "Paid" members thus give another reason for them to subscribe.