That is a good idea; and great photo-manipulation. Is that the liquefy tool?
With the paintings, you might try altering your brush settings a little for easier value transitions; it looks like you've got your opacity set to like 80% and size dynamics set to pen pressure, and the values aren't really blending together. Maybe try setting the opacity to 100%, but turning on opacity jitter and flow jitter and setting them both to pen pressure. Much more like a real paint-brush. (opacity and flow settings are under 'other dynamics' in the brush window, which is F5 by default in ps, I think)
Just an idea. The drawings are good, but I've always found it nearly impossible to render with the brush settings like yours.
I tried to use your tips to create some kind of foreground and add more details to a bland wall. Here's the result:
Nice work. Only suggestion I could offer is, don't be afraid to cut into your picture-plane with big shapes. In your painting, the only big shape is the cave mouth; so the eye goes right to that and then there's nothing else to move it around.
I like your painting style, though; it reminds me of Khang Le's work. Check his site out, if you're not familiar with him; he's a fantastic environment painter:
http://www.khangle.net/I keep meaning to draw some dwarves or something so I have something to post in here. Hmm.