[gchappel]

You never lose the copyright, unless you assign it to someone else.
Do you have your images registered with the copyright office?
"If your photography is not registered with the U.S. Copyright Office prior to an infringement, a photographer may only recover “actual damages” instead of “statutory damages.” "
Most people never register their images, and your ability to legally obtain monetary penalties is limited on non registered images.
You could send the a cease and desist order, and hope they are reasonable to deal with.
No, I am not a lawyer. But I am a fine art photographer and have gone through similar issues in the past.
Good luck
gary