Because it's a charm, and it's magic. It works with bits in the paint--maybe you'd call them mollycules, or whatever--maybe it makes 'em more active--but in the end, it's just magic that makes magic work.

A painting moves because it's been charmed to do it, and painted with the right sort of paint. A photograph moves because it's been developed in the proper solution. You can't think about it in the same way you think about your muggle technology and sciences and all, it doesn't work the same way. It's completely different.