http://www.androidblues.com/cloudtut.html
i hope this link works, he has frames and stuff on his site, lemme know if you can't get to the tutorial... and i'll find another link. he hand painted it,
but there are other ways, I made a cloud brush in like 5 minutes and animated it using flash. It's far from perfect, but it was quick.
smoke-test
I used multiple brush settings, the key is to use the dual brush setting, to help give you variance in the brush's texture.
another option you can go with is to find an already exsisting cloud photo, and modify and clone it and paint over it, until you are happy with your desired look.