WRT copyright, the designs of US stamps issued since 1978 are copyrighted.
Also the US Secret Service website says
Quote:
U.S. Postage Stamps, Foreign Postage Stamps and Revenue Stamps
Printed illustrations of United States and foreign stamps are permissible for any non-fraudulent purpose. Black and white illustrations of uncanceled United States and foreign postage stamps are permissible in any size. Color illustrations of uncanceled United States and foreign postage stamps must be less than three-fourths or more than one and one-half times the size of the genuine stamp. Canceled United States and foreign postage stamps may be of any size whether the illustrations are in color or black and white.
Note: Canceled U.S. and foreign postage stamps must bear an official cancellation mark, i.e., the stamps must have been used for postage. Also, the plates and negatives, including glossy prints, of any United States or foreign obligations must be destroyed after their final use for the purpose for which they were made.
Printed illustrations of United States and foreign revenue stamps are permissible in black and white only. There are no size restrictions for revenue stamps.
The size restriction is rather meaningless in the new world of digital photography. If the width of a stamp is 1 inch, how many pixels does that correspond to?