How can I change my xbox live gamer picture to a picture of a pokemon?

I want my xbox live gamer picture to be a pikachu and I was wondering how to do it. I remember seeing this one guy's gamer picture and it was flapjack, from the misadventures of flapjack on Cartoon Network, and I was wondering how he got that picture.|||It's not something you already have. You can buy/download free gamer pictures in the Marketplace on the Xbox Dashboard. Once you find them you can go to your profile and chose gamer picture, and they will have a grid of all your pictures.|||;)