Am I allowed to pay people using SP for their graphics/artwork to be used on Neopets?

Yes, if you get permission from the artist(s) to use the work on Neopets or other sites. ^_^

Thank you very much! :D

Actually, I think I read somewhere a few days ago that that isn't allowed. I'll try finding that topic.

edit: the search forums option doesn't work at the moment so I can't find it =( If you want to be absolutely sure, I'd suggest filling in a Ticket and ask an UA :)