What would happen if we didnt give them credit though?

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Worst case scenario: They have the right to sue. It depends on the artist; for example, I take stock photography from them all the time, and sometime their terms of use are simple like no commercial use. And sometimes they want pics (usually out of curiousity) to see what their work has been used to make. Its usually pretty blatant what their terms of use are. These people are professionals; you could be using a man's brushes when he's 55 and got a law degree from stanford. I'd give him credit
Credit usually is a nice note in the description area of your piece, not necessarily on the piece. If you have questions most of them have their email address/instant messenger information on the site. Or you could get an account to send them a private note, or ask me to courier a message for you. They aren't mean about it, there is just so much art ripping on the internet that they have to make sure no one rips their stuff.