When an artistic or literary piece is made, an automatic copyright is created for that piece. But when someone takes a copyrighted piece and claims it as their own or edits the copyrighted piece, they are violating someone Else's rights as the original creator. This is called "copyright infringement."
Copyright infringement is financial gain, making a commercial work accessible to members of the public, concealing intent of infringement, providing false copyright, distributing or importing for distribution false copyright information, or removing or altering copyrights. (dAmn-Army Blog: US Law and dA Policy) Not only is it another form of plagiarism, it's a civil violation in the US, where one can be sued for if "Fair Use" cannot be proven, and posting someone Else's work, manipulated/edited or not, in your gallery does not qualify as "Fair Use." When you post something up in your gallery, it is automatically copyrighted to you. However, if you're posting something that someone else has created, the piece is already copyrighted by them. Therefore, by posting something that is not yours, you are claiming the image as yours, even though you may say that you aren't. In simpler words: What you post up here is automatically copyrighted to YOU. If the piece you're posting was created by someone else, then you're altering the original copyright, which is illegal. So unless you have the original creator's permission to post their work or edit it, then don't post it. It's not under "Fair Use" at all.
Interesting fact of thieving/stealing
The only time "Fair Use" applies is if the work you are using, that is not your own, is being used for commentary, criticism, news reporting, research, or teaching. Therefore, you do not need written permission from the original artist. ("Wikipedia: Fair Use") Unfortunately, "Fair Use" does NOT apply to DeviantART, so you will need written permission from the original artist to post, edit, or manipulate their work. ("FAQ #32: Fair Use and Your Submission")
However, if someone files a suit against you and infringement is proven, then it is a violation in U.S. law and the following penalties may apply: ("Shrewmania")
~5 years in prison and/or a fine of up to $500,000.
~Second offenses (or more) can range up to to a fine of US $1,000,000 and up to 10 years of imprisonment. ("Digital Millennium Copyright Act." )
FAQ #8: What are violations of the deviantART copyright policy?
FAQ #20: Where can I find the deviantART Terms of Service?
FAQ #23: What are deviantART's policies on nudity?
FAQ #32: Fair Use and Your Submission
FAQ #63: What sort of things can I use for Preview images?
FAQ #94: Can I use the works which I find on deviantART?
FAQ #157: Can I use things created by other people in my submissions?
FAQ #193: How do I prove that I had permission to use someone's work?
FAQ #217: What are "Stock and Resources" and can I use them in my submissions?
FAQ #242: What are scraps?
FAQ #251: You prohibit the submission of 'pornographic literature'; what do you consider this to be?
FAQ #257: What sort of permission do I need to use someone else's work?
FAQ #304: Do you remove copies and trace-over art?
FAQ #306: Does "Crediting" let me use whatever I want?
FAQ #565: You prohibit the submission of 'pornographic imagery'; what do you consider this to be?
FAQ #572: What does deviantART consider "Fan Art" to be?
FAQ #578: What does deviantART consider to be a proper desktop screenshot?
FAQ #743: Can I sell fan art as prints?
FAQ #752: If my print contains an image that isn't mine, can I still use it for print?
FAQ #809: What are deviantART's stock image guidelines?
FAQ #696: How does deviantART comply with COPPA?