Well actually, if your in the pictures hub, you can long press anywhere that is empty up top where it says pictures for example, and you can custom set a pic, when you do that the pictures hub background along with the tile will always show the selected picture. However, if you choose "change it for me" it'll randomize the pictures it displays in the background and live tile. Mango will bring animation to the tile where it'll pan around the pic while its being displayed on the tile much like the artist artwork on the Zune live tile.