It depends on what program you use. I've never been able to get on with Photoshop but I love PhotoImpact.
Anyways the easiest way I found was to make a circle selection and move it over the planet. If you still struggle I've done it for you - Here's (
http://www.deadlystream.com/gal.tga) the actual icon and here's (
http://www.deadlystream.com/alpha.tga) the alpha channel in a separate file.
--Stream
Thanks it works Ill be sure to mention you in the readme if(when) I release this.