You can do this already, but its very slow.
Image manipulator copy from sprite
For x = 1 to sprite.width
for y =1 to sprite.height
System: ImageManipulator GetAlpha(LoopIndex("x"), LoopIndex("y")) Equal to 0
>sprite add 1 to sprite.privatevariable
Then if you want not transparent just use greater than 0, but beware of semi transparencies.