Maybe the store removed the backing to make it easier for you to apply it to your cake? If this is the case, it sure seems like they would have told you that they had done it.
I have a couple of images that I printed and removed from the backing and then didn't use them, I have kept them in a sealed bag and they seem to be fine, haven't dried out or anything.
Vande3boys, some of the larger in-store bakeries will make the edible images for you, and there are some online supply stores that also sell them. Sugarcraft, and maybe Kopykake as well, I think have a good selection, but haven't looked lately. Like several of the other CC members, I have a Canon printer dedicated to the edible ink, to print my own.