How do you get different gamer pictures for your gamer card

Well i think you can get them of xbox live and if you beat mass effect on insanity or hardcore you get one but i dont know if you have mass effect or if you have the halo 3 collectors or legendary it comes with a few
actually mass effect is a shooter rpg that has two difficulties to unlock, hardcore, and insane. beat them and you will get a n7 gamer pic and a saren gamer pic. Xbox live will let you download some too.