There will come a point when you will ask yourself, "How can I find out who is using my images". Most search engines are keyword searches so you are only going to find your image if a keyword has been used or if the person using your images has linked to you, then you might be able to use a google search, but its not something you can easily do. However there is a search engine called TinEye where you send it your image and it will search for the image without using keywords.
Its now even easier to use TinEye because if you register with them, you can install a plugin for your browser that will allow you to search by right clicking your image and selecting the search item from the menu.
TinEye isn't infallible as it still has billions of images to index, but it is very fast and sometimes the results are amazing. I got TinEye to search for one of my most popular images and it found several matches. One of which was a photo of a painting of my photo on a German website that had sold a limited edition of paintings. Having found this derivative work I could, if I was organized enough, go and get myself a lawyer to sue for copyright infringement. Of course it wouldn't be simple because I would be crossing national boundaries and derivative works aren't the simplest of cases.
One of the things that I believe the TinEye technology will be good for, is as a search engine for orphan works. One of the things that would be required for workable orphan works legislation is a reverse image search engine. If people could seed the TinEye database with their own images, then they would ensure that their images could be discovered on an image search by someone wanting to use work for which they do not know the author. This seeding of the database could be done automatically by sites like flickr when you upload an image. I can't see orphan works legislation being workable unless something like this is put in place
Why don't you give TinEye a go and if you find an image that has been used without your permission tell us about it in the comments.