What do I tip people doing art freebies, and how much? CSC, sP, items? I have absolutely no idea and I don't want to come off as unappreciative.

I don't really think there's a ... rule for this? Tip what you think. If you are absolutely in love with a piece, tip what you might pay for it. If they have a shop for paid commissions, tip some percent of that. Search their wishlists maybe.

There is no rule to do it, it's mostly courtesy. I do so especially when someone was kind enough to hold an item for a day or longer. Art freebies are a bit more liberal than most but should still be tipped. c: