Anybody who has ever been around on the internet knows that the majority of people are not artists. If you spend a lot of time here that might be hard to believe, because here everybody is welcome regardless of skill level when it comes to making content. But on the greater internet true content creators are few and far between, while the people who WANT art are found around every corner of every website. This disparity can prove hazardous to those of us who make art and put it out there, because once it's there it's easy pickings for thieves.
In my time I've found that the worst offenders in terms of unauthorized art usage are roleplaying sites. The vast majority of people who use the likes of roleplay.me, chatango, kik, etc., fall into the category of non-artists. I can't count how many times I've seen people using art they found on Google image searches of "neko" or "cute furry" to represent themselves. This in and of itself is fine I suppose, as they're not claiming the character to be theirs, just using it as a profile. The problem for me comes when people use these characters for their own purposes. When you create your own character based on the image you found on Google you've officially crossed the line into plagiarism. You cannot make somebody else's character into your own just because you like the way it looks. It's one thing to have a preexisting character and to use art similar to what you imagine them to be, but when you claim somebody else's property as your own you lose all respect from me. This is especially disgusting when these people become popular with these stolen characters, which I've seen on more than one occasion.
Too many times I have come across people taking art that I recognize, from people I watch on DA, and saying "This is my original character." Hell I've even seen this happen ON DEVIANTART of all places! How can you be so goddamned lazy not to bother taking five minutes to build your own creation rather than ripping somebody else off?