It appears that you make your own official US Stamps that include a picture of your choice. This is done by using a program called PC postage.
Stamps.com has the following restrictions listed (see below) which appear to be extremely vague. If they are part of a government regulation I would assume they would be void for vagueness (what does otherwise objectionable mean?). Some of them even appear to cross the line being outright unconstitutional. Now, I assume that Stamps.com is a private company. But they also appear to the exclusive way to make photo stamps through the lincesing of PC postage. It appears that the Post Office is getting around the 1st Amendment by licensing.
It appears that there are three vendors. I don’t know if they all require the restrictions, but the law changed to allow ads in the stamps. I am trying to find there are content restrictions in the law.
IANAL, but I don’t think so. They’re basically publishing something, and claiming editorial ability, which is clearly legit.
The government isn’t preventing you from using the government-provided part of the service (i.e. delivery of mail in return for payment of postage): you can get stamps at any post office. So it’s just the value-add portion of being able to create your own that’s in question here, and the government isn’t offering that.
Even if it were, stamps are publicly visible; and are therefore likely subject to the same sort of requirements as signs so far as decency goes. Most of the rest of those limits are just basic copyright and model-release types of limits, which are already controlled by law.
IANAL either, but there was a First Amendment case a number of years ago brought by a guy (or firm) who wanted to buy the adspace of the (now defunct) giant interior billboard in Grand Central Station here in NYC. He was refused the space because the ad conent was deemed unsuitable (obscene, or whatever). I’m pretty sure the court ruled in the guy’s favor.