A Matte Painting,

is a painted representation of a landscape, set, or distant location that allows filmmakers to create the ilusion of an environment that would otherwise be too expensive or imposible to build or visit. Historically, matte painters and film technicians have used various techniques to combine a matte-painted image with the live-action footage. At it's best, depending on the skill levels of the artist and technicians, the effect is "seamless" and creates environments that would be imipossible to film. - Wikipedia