image cropping

[dose]

i know you can crop an image to your sizes, but im tryin to use images like this.

 

http://avatar.xboxlive.com/avatar/Cannibal%20XIII/avatar-body.png

 

The image size itself doesnt change but i wanna know if theres a way to detect the ammount of transparent space at the top of the image.

 

What im trying to do is make a xbox card but the avatar comes out different every time, since the players avatar can range in size.

 

Im not tryin to advertise or anything but heres a better way to explain it.

 

http://yourgamercards.net/xbl/a/Optic%20HECZ.png    that image works fine.

 

http://yourgamercards.net/xbl/a/stuey08.png but this one dont.. cuz the avatar size is different so theres more transparent space.. Any ideas??