In the real world, if your art is awesome, and you distribute it anywhere in publications it will get nabbed. People will save it if you put it anywhere on the internet, and they will even print off copies if they think it's super awesome. Putting up neurotic "do not steal" warnings, threatening viewers with homeland security or the FBI not only makes you look lame, it makes you look ignorant. Copyrights are in place as soon as you create an image. It's not going to stop pirates from stealing your work, either. Ones who do it for a living make their living breaking the law. So while you're doing nothing to deter thieves, you're doing everything to alienate possible fans.
The clever artist will limit damage by limited access to the original. If you don't want to find prints of your original oils, for example, on the streets of Hong Kong on cell phone covers, don't let people have huge images of your work. Don't upload big images unless you want it redistributed.
If you are drawing fanart, you have no right to the copyright. Nada. Claiming copyright or proclaiming that no one can steal your work when it's of Batman or say, The Smurfs, is absurd. You have no claim and no original idea. You are the one who is the thief - but since the "theft" is free advertising for the corporate brand, most companies tolerate it.
A true artist makes his or her way in the world of art by finding their voice. If you are still drawing fan art of World of Warcraft characters or making a DeviantArt account dedicated to some franchise without being hired to do so, you're not becoming a true artist. You're serving someone else's vision. Companies love fan art because it helps their bottom line. You're not doing yourself a favor though.
Be original. Come up with your own dynamic characters and stop following tired visions like some sad media-poisoned sheep.
It's a source of neverending befuddlement to me how *very* many people not only think they know copyright law, but well enough to blather about it to all and sundry - as witness the warnings and threats you cite. Hell, I worked in a COPYRIGHT PRACTICE, and I don't know all the ins and outs of a field so complex that paid specialists routinely disagree.