When you submit a deviation, you have to click a box that verifies that "[You] have read and agree to the Submission Policy and/or contest rules if applicable." And what does it say in that submission policy?
This is the agreement ("Agreement") between you as either an individual artist or as the legal authorized representative of a group artist ("Artist") and deviantART Inc., ("deviantART") concerning the uploading of Artist's audio, visual, audiovisual and other materials (the "Artist Materials" defined below) to the deviantART.com website or any successor(s) websites and/or websites to which the contents in whole or in part of deviantArt.com are assigned (the "deviantART Site(s)") and concerning deviantART's use of Artist Materials on the deviantART Site(s).Condensed down into basic English, it means you are verifying that you are the owner of the content being posted as a deviation. Meaning it's something you made yourself, created out of valid stock material, or commissioned another artist to produce for you.
FAQ #257: What sort of permission do I need to use someone else's work?
DeviantART is a site for posting up artwork of your own creation. It is not a site for posting up scans of comic pages, or screen-captures from movies or TV. It doesn't matter if you worked "hard" to "clean up" the scans or captures, you didn't create it, so you don't deserve to post it.
FAQ #306: Does "Crediting" let me use whatever I want?
The staff running deviantART walk a fine tightrope of legalities to allow us to use this site. They work hard to make sure things are fair and square here. As such, they simply can not allow copywritten material posted without the express permission of the copyright owners. This site shouldn't be abused.
Not to mention it's rather unfair to the real artists on this site, who have to contend with their original material getting drowned out by the clamor of stolen material. (And without legal permission, it IS stolen, even if you don't claim it's yours.)
So don't get pissy if you have scans or screencaps and someone asks you nicely to take them down. Better yet, stop posting up stolen art that isn't yours here and just go use your Photobucket or Facebook or tumblr (or whatever other site is currently trendy) instead.