For decades, art work of the great masters has been used for postal stamp use. The works of Bottecelli, Rubens, Renoir, Titian, Michaelangelo, Gauguin, Raphael, Piccaso to name but a few, have graced the envelopes of letters around the globe. Here is a small sampling from the collection of David Saks.
Topical or thematic stamp collectors develop themes or story lines based on images or subject of the collected stamps. They assemble their collections according to themes instead of country.
Memphis Stamp Collectors Society