Ditto to the guy above...however I happen to be a talented avatar artist. ;D
When dealing with misplaced threads, responding to the thread on-topic instead of just telling them of their mistake is essentially sending the OP the message that it's okay to post in the wrong place. Instead of encouraging rule-breaking behaviour you should either PM the OP, or better yet, report the thread and ignore them.
If you really want to get some gold for your art, go to the art requests forum and find different customers there. They'd probably be better customers anyway xP