The most efficient thing is to send them an email and tell them that you own the copyrighted works and that you demand they stop using them. State specifically which pictures belong to you, and give your name and contact information. Tell exactly where they are using them. For example, is it on a website, on a video, or what? Explain to them exactly which pictures you are claiming as yours. If you falsely make such a claim, you can find legal trouble in return.
Without more details, I cannot really deal with this in detail. If it is important to you, contact a lawyer to help you.
Also, keep in mind, if you created the pictures, they automatically have copyright and unless you were making a work for hire, that copyright will usually belong to you. Therefore, you can demand that others not use the pictures to which you own copyright, even if it is not registered. Registering promptly after creating a work, or before the work is infringed, gives the possibility of certain types of damages in litigation that are not otherwise available.
Answered on Feb 01st, 2013 at 4:21 PM