The service is available via the Internet at www.photostamp.com, and the idea is very cool: you simply upload an image, specify a few details regarding the stamp, and they then generate and mail you the stamps. Neat!
I gave it a try, providing this picture I took on a recent visit to the United Nations headquarters in New York City:
This sculpture, titled "Non-Violence", was created in 1980 as a peace symbol by Swedish artist Carl Fredrik Reutersward. It was presented to the United Nations in 1988 as a gift from the Government of Luxembourg . It's located in the Visitor's Plaza, near the main visitor's entrance to the U.N.
The image was rejected. I had a long conversation with a fellow named Timothy in the customer service department at Stamps.Com, the company providing the service. He said that all photos are cleared through a review committee, and that, as per the site's terms and conditions, images are rejected if they are deemed to be
...obscene, offensive, blasphemous, pornographic, unlawful, deceptive, threatening, menacing, abusive, harmful, an invasion of privacy or publicity rights, supportive of unlawful action, defamatory, libelous, vulgar, illegal or otherwise objectionable...Timothy declined to specify which of these requirements this image failed to meet, but noted that images of weapons are generally rejected.
So - yes, you can get your image on a stamp, as long as it's sufficiently insipid.