Any and all painting and drawing tutorials, classes, photography, etc.

try www.thegnomonworkshop.com and www.lynda.com for training on virtually any piece of software you have an interest in.

The Gnomon site will have MANY for Photoshop and Painter - everything from concept illustration to matte painting to set extensions (the 2nd and 3rd sound more like what you're looking for).

Otherwise, also learn to piece together photographic images, do digital manipulation (PLENTY of books on that), and keep a digital camera with you all the time to shoot anything from trees to mountains, to rocks, to clouds, etc.

And it doesn't have to be the latest greatest high-end $1,500-$20,000 camera - go to www.KEH.com and check out their used digital cameras - you'd be amazed at what you can get for $300.

Good luck - hope that helps-
-Lew ;-)