I question giving ownership interest to artists simply because they are providing artwork. If they are not providing start-up capital, they should not have ownership.
What happens when you get tired of their work? Worse, what happens when your customer gets tired of their work?
Hire artists on a work for hire basis. Here is a sample contract
http://www.creativebusiness.com/pdf_...orkforhire.pdf
They get paid for their work. They can use their work made for you in their portfolio or personal use, but nothing else.
I wouldn't think of having just one or two artists design for me. I love the fact i can use however many and whoever I want to. I think it helps prevent your site from becoming stale.
There is so many quality designs out there from so many artists that we are finding we can get quality designs and pay the artist just based on the number of sales of his design on our clothing.