One thing that artists face who post their images online (and that is just about all working artists these days) is that people will copy their work. As an instructor I allow my students to copy my work for their own use. But it is discouraging when I find other teachers in my very own community who have taken my painting and used it to hold a Paint Party.
If you are working as an artist you need to take the time to understand copyright law. It seems confusing but it’s not at all. “The moment you create ANYTHING visual—paintings, drawings, photographs, sculptures, etc—the only person who is allowed to copy that art is you. If you decide to sell prints of one of your paintings, you can. If anyone else does, without your written permission, you have the right to take them to court and sue for damages.” This copyright information is from Empty Easel March 18, 2008 where you can learn much more about copyright by clicking here.
But I think it is very simple. Don’t steal! If you didn’t create it then ask permission before you paint it if you are going to use it to sell or for profit.