Either because you haven't unlocked them yet but your viewing the item. Or something is wrong with updating your avatar storage space.
Yes
By playing games.
Achievements and avatar awards.
Your Xbox should ask you to delete an avatar when you run out of space for one
yes i need them which games have them
There are no avatar awards for Assassin's Creed II. Sorry.
you a little bit of space for your avatar character
Halo 3 itself does not contain Avatar Awards, but you can download the free "game" Halo Waypoint off of the Xbox LIVE Marketplace. This "game" will count up your achievements and depending on which ones you've unlocked, it will give you Halo themed Avatar Awards.
Yes, Xbox Community Ambassadors get Avatar awards and other things for volunteering their time to assisting other gamers with support issues.
I actually asked the same question. Some arcade games (I think in the marketplace) have avatar awards, such as doom 2. The only game I have awards for are Halo Reach, but most new games have some, just go to 'Edit avatar', go to 'Awards' and see which of your games have them.
No, your avatar can wear whatever you won within any game.