A good rule of thumb if you're going to use any celebrity image is to make sure credit is given to the original photographer and/or a link to their site (if they have one). Also don't hotlink them.
You shouldn't alter any of the celebrity images you use (save for any cropping or color correction if need be). Also, you can't use the (any) celebrity as part of any kind of promotion or sales or any kind of marketing scheme without proper permission or a contract.
If you want to use a celebrity image as part of a blog or some other such ubiquitous posting, it's all fair. Just use common sense.
A google link