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There are Art Shops and Threads dedicated to background creation in the Art Resources section of the Forum. Off the top of my head:
Be sure to read through the information that comes with the images for crediting information. If you are unsure how to or if you should, default to crediting their forum name at the minimum.
Oh, and read through the posts as well as the images on the Official Background Sharing Thread. People have asked and creators have commented their preferred credit type and some don’t require it.