http://www.zazzle.com/jesus+stampsReligious stamps are not prohibited so not sure why this one was the tags may be the reason. That is probably why the Atheist Stamps are allowed also although not to many people's liking. And I remember reading somewhere its not the picture but the word "Jesus" if it were on the stamp but now maybe on the tags and description also.
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Here are the guidelines set forth from the USPS
The following guidelines have been established by the U.S. Postal Service governing images which are restricted from appearing on customized postage:
Content or images, whether appearing individually or in combination with other content or images, that may cause the Customized Postage program to become a public forum, including but not limited to:
Content or images actively advocating or disparaging the religious, political, or legal agenda of any person or entity, including but not limited to content or images designed to influence a specific piece of legislation.
Partisan or political content or images, including but not limited to content or images supporting or opposing election
Content or images related to human reproduction or sexuality, including but not limited to content or images related to services or counseling with regard to pregnancy, abortion, or other sexual matter.
Unlawful content or images including but not limited to:
Deceptive, defamatory, slanderous, libelous or obscene content or images.
Content or images infringing upon the intellectual property rights of any third party, including but not limited to the USPS.
Content or images emulating any form of valid postage indicia or payment of postage.
Content or images that violate any living or deceased person's privacy or publicity rights.
Content or images depicting discrimination on the basis of race, gender, religion, nationality, disability, sexual or political orientation, or age.
Content or images containing non-incidental depiction of firearms, ammunition, or gambling.
Content or images depicting graphic violence.
Content or images depicting illegal activities, illegal substances, and paraphernalia designed for use in connection with illegal substances or activities, or convicted criminals.
Content or images depicting profanity, nudity, or sexually explicit functions or materials.
Content or images related to the sales of alcoholic beverages, tobacco or tobacco related products.
Content or images related to the sale of products designed for use in connection with sexual activity, or product, services or entertainment directed to sexual stimulation.