Excellent question. You can use the copyright image from the stamp company to put over the colored image, or just the circled "C" That way it does not indicate that it is your copyright, but rather just a copyrighted image. If you are indicating the design of you card in whole I always type something at the bottom of the card, which I think is also okay.
However, there is so much stealing images on the websites now. Do you know that people are now not only helping themselves to black and right images from websites but also taking the images that crafters color and lifting them and saving them to their computer and using them as colored clipart. In fact if you contacted the stamp company they would likely provide you with a free digital copyright symbol.