I had a similar situation and I actually wrote a letter to the "flat fee" listing broker that they owed me money to compensate me for using my images. I asked for $1k. Their attorneys responded and although I didn't get any money because I didn't pursue it further, my images were no longer displayed on that broker's listing. They "claimed" the images were provided by the Seller and they had no idea where the images came from.
Good luck... send a letter to their broker of record and make them remove your images and also pay you for the time they used your images without your consent! $1,000 seems reasonable.