If you’re trying to pull just the avatar from within a loop, this shows how to do that:
http://wpmu.org/how-to-create-a-gallery-of-members-avatars-in-buddypress/
If you’re trying to pull up someone’s avatar simply by ID, use bp_core_fetch_avatar() like this:
<?php
global $bp;
$the_avatar = $bp->displayed_user->id; //you'd put in whatever function you're using to get the ID here, assign it to a variable
echo bp_core_fetch_avatar( 'item_id='.$the_avatar );
?>
Look up bp_core_fetch_avatar() as there are a lot more options than this. For example, do you want a thumbnail or full sized image? There is a way to specify. Hope that helps!!