Fluffy cloud images created from actual photographs.
The background is a uniform color, but the gimp file is layered and the cloud is alpha blended so the cloud can be extracted and pasted on to any similar sky blue color and still look good, so feel free to toss it into your own sky images.
The actual sky has a natural gradient, where it's typically brighter at the horizon, and darker straight up (because you have to pass through more atmosphere to get to space when going at the horizon than you do when going straight up). Because they're alpha mapped, these clouds should still look good even if you put them down on a sky gradient, so you could potentially get some very realistic looking sky by pasting these clouds around.
Feel free to create backdrops with them and post the results to the OGA with the same licenses :)