A background image is an image that is projected onto an orthographic viewport to aid in creating a model. It helps the user with getting the scale and shape of meshes right by providing a visual reference. A different background image may be applied to each of the 6 pre-defined orthographic viewport directions.
A background image can by assigned to a viewport direction using the Select Background Image Window. After opening this window, select the tab for the direction you want, and then click the "File" button to select an image file. The image file can be any format supported by MM3D. Background image dimensions do not need to be powers of two.
You can move and scale a background image to position it where you need it for modeling purposes. Use the move and scale background image tools for this purpose. Scaling always preserves the aspect ratio.