You must obtain written permission from the proper and legal owner of any work which you wish to use, credit alone does not replace this requirement.
Official written permission or license is required whenever you wish to use artwork or photographs created by someone else; it is also required if you wish to use screenshots, official artwork from a video game, scans from an art book, or any other source material which is considered the legal property of another person. Permission is still required even if you have planned to manipulate, color over, blend the original with other images, or otherwise digitally work over the original work.
Explicit permission is not required when you choose to use valid stock resources. Please check your sources carefully to ensure that they are valid stock.
"Fair Use" is the notion that some public and private uses of copyrighted works should not require the permission of a copyright owner.
These circumstances are very limited, complex to analyze under the law, require the help of expert advice from a lawyer and are ultimately only determined by judgment in a court of law.
Since the subject of "Fair Use" is widely misunderstood, misinterpreted and misused as a defense deviantART reserves the right to reject any submission which makes use of content by claiming the "Fair Use" exemption.