A very great reminder to artists out there that the work they post isn't entirely safe from thieving hands. Very well said. Your art and other projects are your responsibility always, and unless you want someone stealing your hard work, I suggest putting a copyright on the art itself where it can't be easily cropped or edited out. DeviantART has their watermark and while it may not be the prettiest thing in the world, it could very well help. There are people out there willing to overcome those boundaries and discard your efforts of keeping your work safe, however, and there's simply nothing that can be done about it aside from reporting and safely removing them from the website or other social media. Always keep an eye out for stolen art, and if you see work that does not belong to the uploader, notify the original owner or report immediately.
Like Turtlex, I see this happen quite frequently. Whether it's from jealousy, laziness, or the urge to just be plain rude, stealing someone's work and making money from it is by no means okay. Giving viruses to others isn't okay. Creating a website that ensures both is absolutely horrid. Stay away from those type of websites and spread the word if you happen to come across one. Do not involve yourself in the matter by directly speaking to the people behind it. Drama would most likely ensue and nothing is gained from it aside from a heated argument. Thieves are more likely to deny the fact that they stole the art or they might just be stubborn throughout the situation regardless of how you put things. Report the site, the people, and grab others who are willing to do the same in an attempt to remove them from the internet.
Overall, just be safe. Thank you for posting this, Finni. I hope these sort of things get resolved soon. It truly is a disgusting sight to see and no artist deserves the trouble in having to deal with an art thief or other disrupting sites.