Create a new image, 400x400 px.

Select a cyan-like color for your foreground color, and then a cyan/blue color for your background color.

CTRL+Shift+N (create a new layer)

Filter>Render>Difference Clouds

CTRL+F a few times till it looks good.

If you want, you can now run over it a few times with white as your foreground color with the airbrush (just hit "J" on your keyboard to summon it.)

Filter>Render>Lens Flare

You may choose any lens flare you like. 130% is generally a good percentage to have it on. Click anywhere on the thumbnail to place where you want the lens flare (the sun, in other words.)

There you have it, a sky for a background. Now, you can put whatever you want on top of it. Have fun! :^)