Understandably a very aggravating situation. Copyright is yours automatically as the creator of the image. The right, and expense, to defend said copyright is also yours. Financially it is usually a loss for all but the lawyers to try to sue over wrongful use. One tactic that many folks have used successfully is to invoice the folks who are using the images wrongfully with the option of paying or to cease and desist. As wrong as it is, if they refuse to pay up or take them down there is very little you can do about it other than to accept it or go the lawsuit route knowing that it will cost you money, along with a measurable portion of your lifespan, in the end and even if you do win then you still have to deal with collection which can be another round of aggravation for you.
I wish you well with this and hope that you are able to resolve the situation in a friendly manner.