Well a quick search in google turned up with a bunch of vids, and a couple of other sites that look like they could help.
Here: * Ok the hyperlink isn't working. Just type:
"Watch and learn digital painting" into Google.
Also try Gnomon. They have a bunch of great concept painting DVDs but you will have to pay for them.
www.stevenegan-cgi.com
"Your weapons are no match for ours! People of Mars, surrender!"
"Um, this isn't Mars. This is Earth."
"Earth? Earth-with-nuclear-weapons Earth?"
"Yes."
[long pause] "Friend!!"