Indeed the internet has opened up a can of worms as far as copyright infringement is concerned.
It makes it very easy to get pictures & artwork, and unless you can prove how much someone actually made off your work, there is no point in a lawsuit. Especially if the work wasn't copyrighted (sent in copies, with appropriate forms, etc. NOT just putting "MINE" on it with a C, the date & your name).
That is why it is done so freely now-a-days.
And with the internet, it is now assumed that if things are posted or placed in public exposure, they become open for public use. That is NOT in the best interest of the creator by any means. (More importantly, any information you place on the net, even private information, is slowly being accepted as public knowledge, no longer private!) Everything! What you write, what you buy, where who, when!
(This gets back to the posts where it was discussed about us losing our rights, without our even being aware... but I will stop here on that.)
Anyone who has gotten a bootleg CD, DVD or video game, a nock-off purse, shoes or other items, or even a home made copy of some songs made from some old CD's, is actually partaking in copyright infringement. (As was brought up about using a work for an avatar is included. For how would it be if someone used your picture of your shrooms or one of your CP's as their avatar without mentioning that it was yours? From the look of it, it doesn't feel good to be on the losing/being used/bad side of that no matter what the situation. Lack of permission is lack of respect.)
While such things are a compliment in a way, they are also theft. They show disrespect.
For as much as most of us wouldn't mind the use if the person (thief) would simply ask if they could use it, that seldom happens for instances like this.
I am 100% with Whimgrinder on this. Spot-on!
As an artist, designer, etc., I have also repeatedly endured illegal use of my work, and it never evokes good feelings!
When someone actually respects you & your work however, and works with you & communicates openly, that is a totally different story!
(Thank you Ryan for re-establishing my faith in people!)
You can get them to stop using your work (possibly), but to get any money from them at this point is unlikely. (Especially of they are not making any money off your work, there is no "valuse" to its use & hence there are no damages to reward you with.)
Your best possible solution might be to contact them & tell them that you noticed that they are using your photograph, and that you would appreciate them posting that you are the creator of the piece. With that they are welcome to continue using it. If they give you a hassle, you will then know what kind of people they are, and at that point you could try to get them to stop using it altogether.
It is a difficult place to be, having to hide your works, creations & ideas... but many of us have been there, lest someone take our works as their own. It gets even better when someone takes it & changes it a little, hoping to make it their own! Talk about devious! But then back when I did electro-mechanical product design work, the corporations would do top secret research, which turned out to be nothing more than a room devoted to taking some competitor's product & canibalizing it, and changing what they find just enough, to make it their own (without infringing any copyrights) but still getting it to work... somehow!
Talk about research! Ha!
What a bunch of BS! But then, this is typical! Sad, but typical!
By the way, whatever stolen avatar that was being referred to, perhaps it would be wise to not do unto others, what upsets us when they do it unto us! Can we rightfully make others make notation or stop using our work, if we are not willing to also do the same.
(I am not sure what work/avatar was being spoken of by this, but indeed it is a bit of a double standard.)