There are some picture packs that can only be accessed with a code from the Official Xbox Magazine. There's a Fallout: New Vegas one that I want that I'm having a very tough time finding. Others are the ones you get for doing something in-game, which at times can be kind of hard.
I have the Xbox Staff gamerpic that I have never seen a single person have on to this day.
Also the V1 Xbox pic is kind of rare, and I though custom ones you could only see if your the one who has it on?