If you're taking the picture from a website, then the picture will likely be copyrighted, to either the site, or the photographer, or even possibly the celeb themself. Unless the picture is explicitly described as royalty or copyright free, then you have to assume it isn't (and since it is a picture of a celeb, it most likely won't be).
You will need to ask wherever you are taking picture from for the rights to use it, and they are perfectly within heir rights to charge you a fee to do so.
Your only other option is to go out with your own camera, and take a picture of the celeb in a public place. It has to be a public place for you to be able to take their picture legally without asking them (at least, that's how that part of the law works in the UK, if I remember correctly).