Legal Question in Intellectual Property in California

In most cases, it will be a violation of copyright laws to post the picture. The issue is not that the celebrity is a famous person but rather the fact that someone else took the photo. If you took a photo of a famous person then you could use that photo; however, if somebody else took the photo then no, you can't use it unless the photo is in the public domain or in something called the "creative commons". Lots of people who run blogs are in violation of copyright and don't even know it. They have opened themselves up to a lawsuit if the original photographer ever decided to sue.

You should ask for and receive written permission from the copyright owner. Otherwise, you are probably infringing the copyright. The chances of getting discovered and sued vary, but are not zero.