I found one of my favorite images posted by an unknown person without permission, on this forum, [link] & hosted by image shack.
I contacted Image Shack staff, reported the violation and requested that my photo be deleted. I'll let you know what happens. In the meantime, see if your images are posted by this user. I think I recognize several pieces by other Deviant Artists. Please feel free to share this with your friends.

Here's how I found out:

Go to google.com, click on images in the top menu. Open up another tab in the same window, navigate to an image that you want to search for that's online, left click on it and drag it to the google.com tab. drop the picture onto the search bar and hit enter! It'll search for photos that are similar and let you know where it is posted online.
It is NOT infallible but I've already found one of my images being used without permission.
Another tool for this is here:[link]
I haven't tried it yet...