using the magic wand is a horrible way to render though..leaving it really choppy, there is a more precice method using teh pen tool, but for you i would stick with the magic wand like he said.
But if you got that image off of Planetrenders, then i would suggest you make an account...instead of saving the image preview (which that looks like) if you have an account you can click the image preview and make it fully transparent and most of the time alot larger and better quality too.