I thought I'd talk a little about this, after reading a few people who were genuinely distressed and upset that their desire to share something they find beautiful was met with hostility from the artist. But the way this work was shared was... saving the artwork and then uploading it to their deviantart gallery/websites and they didn't understand WHY the artist was upset. I'm not sure I can fully clarify, but I can at least give my thoughts on it and things that I've gone through when dealing with people using my work without permission and hopefully that helps a little with understanding.
The first thing to remember is that doing the resear