An interesting question and, at least internationally, the answer may vary from country to country, depending on their laws.
As far as the recent announcement by the US Postal Service about putting living people on stamps, and given the litigious society in which we live, I would imagine that there would be a specific written legal agreement in place before the issuance of any such stamp.
Actually, the US Postal Service has been in litigation before as it relates to US Postage Stamps, not necessarily depicting one's image, but using a copyrighted image created by another, as in the 37-cent Korean War Memorial Stamp. In this case, the USPS paid a $5000 royalty to the owner of the image used. Here's the court case, if you want the read the "legalese":
http://www.oblon.com/sites/default/...20States.pdf