I knocked this one up for fun. Actually I was just studying one of my cloud photos and analysing the colours by sampling them and trying to paint a similar cloud in Photoshop. I hadn’t intended to do more than one or two clouds but couldn’t stop.
I used the eyedropper to sample key colours off the photo then painted them in on a blank 520×520 pixel image. I used a Wacom tablet and pen with Photoshop CS on a Mac. It would be frustration-plus to try it with a mouse since the pen gives you interactive control over brush size and opacity simply by adjusting the pressure on the pen. This makes “whispy” strokes easy.
This is pretty rough and represents about an hour’s work. It’s more an exercise in reproducing photographic effects than an exercise in art.
Although I use computers in my “day job” as a graphic artist, I don’t generally use it to produce this sort of artwork.