I dont think the ToS says anything about it, but i'd double check. I personally see no problem with it, and i think people would happily commision you. If i were a better artist i'd think about it myself, far better than crafting for sure X) If worst comes to worst you'd *probably* just get a warning if SE doesn't want you to do it, especially if you looked in the ToS and forums for an answer. Im not sure how heavy SE is with the ban hammer.

Good luck with finding your answer, and making some gil.