The only thing that would take your copyright away is if you signed a contract giving them ownership of the image. The only other argument was that you were paid by them to create the image for them, which you say is not the case.

A lot of times for web content, I'll sell people an image for "x" amount of months/years. Meaning I stop selling that image to other people for x amount of time, but after that time, they must take it down from their website.