I know we're being trolled, but if you want a serious response I'll give it a go.
It all depends on how the image was used. A few points to consider...
All works are automatically copyrighted at the time they are created. That copyright does not need to be officially registered for you to enjoy copyright protection. You made it, you own it, baring the presence of a contract (such as employment) stating you are creating said work for another party.
Many of the sharing sights, free cloud storage sites, photo contests, etc, have terms of use that you have to agree to in order to use their service. It's common for them to insert clauses that give them usage rights to the image. They can even be very comprehensive usage rights in some cases.
That said, the simple act of publishing a work, on the Internet or otherwise, does not automatically make it public domain or free for anyone to take and use anyway they would like. There is what is known as "fair use" which usually extends to stuff like editorial use, satire, educational use, using portions of a work for commentary and such.
So the short answer is yes you can sue. All you need is a lawyer nutty enough to take the case or the ability to file the suit yourself. Will you win vast sums of money? Probably not.
It all depends on how and where the image was used. So give us the specifics or STOP TROLLING!