Thanks Muireadhach and Laurels! And thanks, Xtreemli, for the feature :)

Reylari, I did it from scratch with a wacom; I'm not much good at lifting images from photos and making something look good from that, though funnily, that's what I started out doing at first, heh.

After a rough sketch for the layout and composition, I fill in big blobs of colour, and add shadows to give the objects volume so as to not appear so flat... after that, it's mainly refining down again and again... I find that it's not unlike real painting, as in, if you have a decent grasp of form (ie, how to render objects with light and shadow), the detail and texture comes naturally enough after that, it's just that it feels less natural because you're not painting directly onto a tablet ;).

I don't have a step-by-step for this last cave painting, but I do have one for a pond scene I did a while back:
http://geocities.com/fizzii_ish/sierratutorials/pond.html

I do use a couple of custom brushes as well, though technically, anything could be done on computer with any brush. Usually, it's easier to blend colours with a speckled brush (I use Photoshop), rather than the standard hard round brush, because it behaves more like a natural brush, whereas the round brush has hard edges, and the fuzzy round brush makes everything blurry (and hence, more prone to flatness, since it behaves like an airbrush). For rocks, I tend to use the charcoal smear brush in Photoshop, as it allows for placement of two-colours at once, to give the appearance of texture. And sometimes I use the leaf brushes for foliage, though these only work well if you know where to put the shadows.

There's not much else to say really... painting basically comes down to some observation and lots of practice, like most things.