Its not a big deal.... Until its you. Until your own pictures and/or OCs get stolen, traced, drawn, written, or used without your permission. Then its a big deal... But you don't know what to do... You just know you are very mad, and what they did needs to stop. So come to me and your other friends and other people who you know have experience and ask for help.
Some people may think, well, they like someone else's OC and don't have their own so they want to use yours. Its even easier if you're famous because this person feels like you won't notice they took it. Their friends will praise them for their great art and/or OC, but you know its yours. They may try to justify themselves by turning the blame "you shouldn't have put it out there if you were so weird about other people using it". Or, "its just a fanart/fanfiction of your oc". If its not yours, how hard is it to freaking ASK for permission to draw someone else's character? Not hard at all. In fact most people will grant permissio