To start I'm going to do a quick tutorial on what I got a lot of flickrmail over, clouds:
First, of course, is how I got the clouds. It took me a long time to work out, but I use the cotton stuffing from one of my pillows, there might be other things that you can use so experiment.
To start I took a photo of the wall that would be my background, with the exposure at -2. I then took the photo of myself, using a remote and tripod (I dont know how I used to cope without!). Keeping the camera in the same place, I took two other pictures, holding two separate chunks of cotton in the sunlight, keeping my hands hidden.
These are the main 3 shots I used.
Using basic layering in Photoshop/GIMP you can arrange the shots together and crop until you have something you're happy with. Generally I find that 'clouds' tend to blur together easily anyway, make sure you use the background image as well to fill in any huge gaps.
After that use the clone stamp tool (I dont know what the GIMP equivalent is) and dodging and burning as required until you get the desired atmosphere :)
