This is quite an interesting topic because many of us will get into rage mode when we discover unauthorized uploaded art. There is many ways to deal with this, here's some of my suggestion if this ever occurs.Stolen?
Some DA users are brand new to the site and do not fully understand what they can submit. They may treat it as a "this is what I like" site by uploading their favorite art. Nowhere in the description they claim they made it, but too quickly we will yell out "THEIF!!!" when the user had no intention. This is how you handle this situation...Comment on the artwork kindly explaining that you cannot upload the artwork without the artist's permission. Or ask the DA user to leave a source link and credit the artist.Stolen!
The biggest sign something WAS stolen is.... 1: They do claim they are the ones who painted it, when you know it was not. 2: They are selling it without the artists approval using DA's selling system. This is how you handle the situation.Don't comment! It does nothing to improve the situation. Report to DA about the stolen art, they do care about this kind of stolen art. Notify the original artist about the stolen artwork.
And that's it! So if you come across artwork that belongs to someone else, just follow the above guidelines.