Often the "CGI" background is a combination of "true" CGI, manipulated photos/footage, and digital "hand" painting (at least for the deep backgrounds). It's muc

Often the "CGI" background is a combination of "true" CGI, manipulated photos/footage, and digital "hand" painting (at least for the deep backgrounds). It's much more flexible than physical painting. For one thing, there's undo, and versioning, not having to wait for paint to dry, etc etc. For example, if you want a castle in the mountains in the background, the mountains might be a composite of a photo, and some extra CG. A castle is then rendered "in the rough" ontop of the mountain, and then details hand painted in. And then the whole thing gets another CG/painting treatment to make sure everything fits in and to create more atmosphere. Every matte painting is different. If you're interested, you can poke around cgsociety's (sorry it's blocked at work) forums in the digital matte painting section. It's really just a huge grab bag of techniques.