Only you can answer the question "Is it a good idea?" - It's a classic risk/reward decision. Similar to "Is it a good idea to go 10 mph over the speed limit?" You will be in the wrong and you will be hoping you don't get caught. It's a calculated risk. What is the risk of getting caught? Maybe not much. Who knows? But the more you have to lose, then the less inclined you should be to do it
My advice would be to try to get an official photograph, one that is in the public domain or one that the star has given a blanket permission for. Try wikipedia for example or the celebrity's official press office. You'll be surprised - sometimes you can get official photos with permission. And in the worst case scenario, if you decide you will have to *cough* use a photo you don't have permission for, then try to fly under the radar as much as possible (don't use a rare exclusive photo for example...) but be aware that officially you are risking getting a strike.
Good luck