This postcard has an odd combination of stamps and cancels. The stamp that was placed on the picture is a 1978 Christmas stamp. The stamp on the bottom in the white area is a 1979 Christmas stamp with an official First Day of Issue cancellation. I am guessing that a 1979 maximum card was made first, and the 1978 stamp was added later with an unofficial cachet commemorating the visit of Pope John Paul II to Washington, D. C. Both stamps were rather sloppily applied, with the 1979 stamp being damaged before it was cancelled.
The 1978 Christmas stamp has an image of "Madonna and Child with Cherubim," c. 1485, by Andrea della Robbia. This is one of the few Christmas issues reproduced from a sculpture. The original is glazed terracotta and is in the Andrew W. Mellon Collection, National Gallery of Art.
The 1979 Christmas stamp has a simplified detail from a painting by late 15th and early 16th century Netherlandish painter Gerard David. The original painting titled "The Rest on the Flight into Egypt" is also in the Andrew W. Mellon Collection, National Gallery of Art.
This is a post for Sunday Stamps at Viridian's Postcard Blog