Using the terraformed Mars map from wormspeaker's thread her is my attempt at a planet.
In gimp i used "scale image" to make it squre. Then ran "Filters > Map > Map Object" selecting Sphere from the menu to make a globe. I played around with it until I had the view of the map i wanted.
Then I ran drop shadow twice with a blue color. I adjusted the dropshadow layers transparency till it looked like air.
For the clouds i made some light blue splotches an right side of a trans parent layer, then used the smudge tool liberally on the edges and made them all cloud like. Then once again ran the "Filters > Map > Map Object" tool to make it all spherical and then placed that layer on top of the planet and adjusted it opacity till it looked right.