I used the eyedropper to sample key colours off the photo then painted them in on a blank 520x520 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.
(Clouds over the Stirlings. Digital painting, 2010. © Andy Dolphin)
Although I use computers in my "day job" as a graphic artist, I don't generally use it to produce this sort of artwork.