How to get Call of Duty 4: Modern Warfare prestige icons as gamer pictures for Xbox Live?

I see some of the people on my friends list that have CoD 4 prestige icons as there gamer pictures, and I'm wondering how they got them. I think it's because they were free for a limited amount of time in the marketplace but I'm not completely sure. Can someone clarify?|||you buy them in the marketplace|||go to xbox live market place and look for call of duty 4 look under picture packs and it should say some where call of duty 4 prestige icon picture pack