That's the problem with the internet: you can upload whatever you want, but people can download whatever they want. That's how it works.
It has laws of course, but they don't apply to somebody who has decided to publish something, knowing that everybody is allowed to download it.
Same happens with wallpapers, and random images from the internet.
Another different thing is that you have a copyright and they're earning money with the art.
I've had the same conversation not so long ago, related to "this person clearly writes "I've done this, look at my art"". You can see pretty clear when somebody hasn't created something. You don't need that much brain to know that xD
And the person you showed me CLEARLY hasn't created any of the images. Though if what you're asking for is that Twitter delete those images... you can sit down and relax. Maybe they will do it once, but not more than that.
And what you're complaining about is human nature. Unless they know and respect that there are other people than themselves in this world, that's what they're gonna do: whatever they want
I've dealt with an art thief myself some years ago, know their attitude: human nature. That's all