I enjoy art on all types of Christmas cards, and no matter what genre they are, when they're done well, I can really relate to it. But possibly my favorite art on cards comes from the images on vintage Christmas cards, which not only sent a message.
Classic train christmas cards love this
Victorian Christmas postcard, holly lady with fur muff.
A "Merry Christmas" antique postcard.with "Holly"
bumble button: Beautiful Antique Postcards featuring Sprigs of Holly Más