If anyone wants specifics, look up California Civil Code seciton 3344. Here's paragraph a)
'3344. Use of Another's Name, Voice, Signature, Photograph, or Likeness in Advertising or Soliciting Without Prior Consent.
(a) Any person who knowingly uses another's name, voice, signature, photograph, or likeness, in any manner on or in products, merchandise, or goods, or for purposes of advertising or selling, or soliciting purchases of products, merchandise, goods or services, without such person's prior consent, or, in the case of a minor, the prior consent of his parent or legal guardian, shall be liable for any damages sustained by the person or persons injured as a result thereof. In addition, in any action brought under this section, the person who violated the section shall be liable to the injured party or parties in an amount equal to the greater of seven hundred fifty dollars ($750) or the actual damages suffered by him or her as a result of the unauthorized use, and any profits from the unauthorized use that are attributable to the use and are not taken into account in computing the actual damages. In establishing such profits, the injured party or parties are required to prove his or her deductible expenses. Punitive damages may also be awarded to the injured party or parties. The prevailing party in any action under this section shall also be entitled to attorney's fees and costs.'
There's also a lot of case law, such as Kareem Abdul-Jabbar v. General Motors Corp., 85 F.3d 407 (9th Cir. 1996).
The federal Lanham Act may come into play too.
As others have mentioned, one of the main keys is whether it is for commercial purposes. Otherwise it can very well be Fair Use. But the bottom line is, if the people violating these provisions don't have deep pockets, there is not much to be done about it.