Arago: People, Postage, and the Post to view images of United States postage stamps.
For Christmas and other holiday stamps, search "holiday" in the category, philately. You can see a stamp--enlarging it for detailed viewing--, and you'll find information about its history, with design details. In addition, "The Art of Christmas Stamps" is among Arago's many online exhibits.
Arago is the website of the National Postage Museum, part of the Smithsonian Institution.