Rendering an image to part of the screen?

Is there a way to have an image be rendered but only fill a part of the screen?

The complete effect I would like to do is to have a wallpaper graphic fill the entire screen, and then draw the game or normal render on top of that, but constrain the rendered image to a particular size/aspect ratio. The best comparison I can make is how modern ports of old arcade games are done; they draw a wallpaper behind the actual game so it can be letterboxed/pillboxed with a pretty image, and the game screen can be adjusted to various sizes according to player tastes.

Additionally, is it possible to have that rendered game image be rendered at a specific desired resolution, and then have that image scaled onto the screen?