I would say that it’s a gray area.

- Doing commissions for real money or Gil is fine.

- Advertising the prices in shout chat, or forums is not fine.

- Shouting invites to a stream however I have seen, and as long as you’re not obnoxious, people do this regularly. Usually through a linkshell.

- Posting fan art in the forums might be allowed. Square Enix does host yearly art contests. Perhaps the easiest way would be to make some kind of fan art thread, so that it benefits all artists. I see draw your own character posts once in awhile.