Advice for those editing cloud textures:
Change the transparency color to black in your image editing program. This makes it easier to see the white clouds, and in particular spot posterization. On GIMP this can be found under Preferences->Display
* Display a grid of the appropriate dimensions to ensure you don't draw clouds over the texture boundaries. Otherwise the clouds can bleed into each other and you can see nasty vertical and horizontal edges.
* Use a white airbrush and a smudge tool to make the cloud textures look random.