Art theft occurs unbelievably often, but even if it happened only once every two hundred years, art theft is absolutely appalling no matter what. The thieves range from immature and stupid to ignorant and stupid, making battling art theft an exhausting and often frustrating task. On DeviantArt or most other art sharing sites, you have the option of simply reporting the stolen art, however that is not the case for other personal or even commercial websites. You might be a DA user, but your art could be distributed in other places too.
Now, it has come to my attention that half the theft occurs not just because people don't give a damn, but because the artist hasn't bothered to properly protect his or her work.
Thankfully we've come quite a ways with Copyrights, but it's also something that a lot of people remain ignorant towards, even the artists themselves. It surprises me time and again how unaware a lot of artists are of the rights to their own work and what they can do to first of all prevent theft, then stop it--you all have a lot more rights than you think and it starts with the fact that as soon as you create something, it is automatically copyrighted to you.
This is obviously false. But if you as an artist do not have a disclaimer or at least a Copyright statement somewhere in plain view, then the thief can very easily feign ignorance. Of course ignorance does not excuse simply taking something that doesn't belong to you (see: theft), however having the thief feign ignorance is something that is, I believe, best avoided by placing a simple "Copyright © year-current-name-. Do not use". Sweet, short and to the point.
Will that stop people from stealing? Of course not. The sad truth is that there are many people out there who just don't care, as I've said before. Fighting those becomes unavoidable at some point because they're just hell bent on stealing. That however does not mean there is nothing you can do to prevent at least some theft from occurring, or ending up losing the battle if it does happen. Here are some things I've done that have really helped me. I'd say they're tips:
Add your signature. Either cleverly hide it within the artwork/sprite or put it in plain sight, but place it on the work.
Watermarks. Yes, they're hideous, but recommended if you're going to hand out a high resolution work. Or in animated sprites, sometimes I'll add a watermark in the middle of a particular frame, for example.
Write a short disclaimer to place somewhere near your work. You should let your viewers/fans/whoever know if you're okay with your work being distributed, edited, reproduced, sold, etc. Tell them they need to ask for permission first. Surprisingly, some people actually do ask first.
When someone contacts you asking for use of your work, do not give them a definite answer right away. Ask for a URL leading to the site they wish to use your work on (you have a right to know this, as well as what they wish to do with your art), make sure it's valid, then give your answer. If you say no, some people will use it anyway, but now that you've got their link you can check up on it if you care enough.
Create work-in-progress shots. No better proof than that, just in case ownership is ever called into question.
Keep all original files and layers if you have them. Again, if ownership is called into question, the best proof is a work that's not only in it's original size, but has all the layers and what not. A superb idea would be to keep back-ups of all your filessomething you should do anyway, regardless the situation. But do not think you owe it to everyone to give away proof of ownership. Once that proof is out there, it's out there (one of the reasons why I'm hesitant with sharing WIPs).
Never, ever display work in its original size. You cannot take a smaller image of say, 400x300 and blow it up to 4000x3000 and have it look good. That your work looks good at a high resolution is your proof that it's yours. Therefore it is best to lower the resolution and distribute only that version online. This, incidentally, also makes it harder for people to use your work in things like wallpapers or what have you, since having only a low res to work with makes for a hideous piece. =]
Always, always keep exchanges made between you and the persons wishing to use your work or having stolen your work after you confront them. Sometimes third parties will have to get involved, such as if you're trying to have your art removed from a website, you have to contact the website host and include the exchanges you had with whoever is giving you grief. Do not rely on e-mail alone or whatever method you used to contact the person. E-mails can get lost, your account can get hacked, etc. so take screenshots (and make sure you have the date on there), save the web pages (if it's on a forum and the moderators decide to delete it), chat sessions, etc. and keep them in properly labelled folders for future reference. Refrain from modifying these files, remember that the date of creation and date when last modified are in the file's properties (right-click, Properties).
A common belief is that the Poor Man's Copyright is a good substitute for registering or keeping screenshots, etc. It is not. Remember that envelopes can be tampered with, so all the PMC gives you is a date, which is great, but it does not proof ownership so do not rely on it alone.
Finally, make sure you're specific in what you are and aren't okay with. For example, you can say "not to use for commercial purposes", but that doesn't necessarily mean the person shouldn't resell the image to a third party once they're done with it. Fine line, of course, but you have to remember that when caught doing something wrong, natural instinct is for people to find ways to pardon their behaviour (i.e they'll look for loopholes in your agreement).
If you find someone who has stolen your work, your first step should be to politely contact the art thief. It's very likely the person is not aware of the illegal use of your work. Images distributed online tend to go around, especially the ones that appeal to the greater masses. It is your duty to present the person with a cease and desist letter, it won't help your case at all if you try to get on a personal level with them or worse, end up being rude.
"I've been made aware of the unauthorized use of my artwork on your website. I request the removal of insert image title here--, which belongs to me, -name here-, and your use of it is infringing upon my Copyrights as clearly stated on my website: -URL here-."
"You stole my artwork you piece of shit thief. Take it down or I'll take legal action!!!"
(Please note, that is not an example of what a cease and desist letter should read, but it is an example of what you could say if you do not wish to send a cease and desist.)
The desire to trash talk thieves can sometimes be nearly uncontrollable. Unfortunately doing so can only hurt you. The same goes for making a public statement and getting your fans to pester the person guilty of theft, which can and will be considered harassment. Only the original artist should contact the person, however if the artist is no longer around, simply advise the thief that what he or she is doing is in fact Copyright infringement and ask them to respect the artist by removing the work. But again, refrain from trash talking the person and harassing him/her by e-mailing them on different e-mail addresses, stalking them on forums, websites, journals, etc. even if you're just defending an artist. I hate to say it, but it just makes the artist look bad. =/
My recommendation would be for you to get in touch with the artist, if at all possible and to notify them of the theft, then leave the rest up to the artist him/herself. If your own work has been stolen then you ought to send a cease and desist letter straight away. You are obligated to give the person 72 hours to comply, after that, no you don't need to take legal action, you can just get in touch with the site's host and ask for assistance in the removal of your work from the offending site. Provide proof that you've contacted the person, also bear in mind the Digital Millennium Act (where you will have to make a couple of statements, provide a signature swearing that you're the owner, etc. If anyone needs help with this I'll be glad to lend a hand), it also helps to provide proof of ownership, just to give the host more encouragement to take action.
Regardless of what you do however, you need to know that art theft can't be avoided entirely. I've said it twice now and I'll say it again, a lot of people don't care at all about rights and regrettably also know that not everyone can afford a lawyer. But I'd say that it's extremely irresponsible for any artist not to cover himself or herself as best as possible. For the record, protecting your work has nothing to do with arrogance as I've seen so many people accuse artists of before. I've seen some artworks stolen that were, quite frankly, terrible.
And lastly, for anyone who believes that if you don't want your work stolen you shouldn't post it online: That's just about the dumbest, most pitiful justification I've heard. People are just as likely to steal art from magazines and books as they are from the net. Does that mean artists shouldn't submit their work to magazines? Does that mean art books shouldn't have any artwork? Same goes for items displayed in a shop, for example. Don't display the merchandise if you don't want it stolen?
Following your logic, no artist should display their work unless they really don't care about it. Which would bring about a fairly swift death to websites like DeviantArt.com and defeat the purpose of personal online portfolios, wouldn't you say?
And that's all I have to say about that.