A quick way to make a JPG image of your drawing is to use the JPGOUT command.  This command will use the extents of the drawing window or a selection set of objects to create an image on a designated drive.  The background color will be honored.  Since most of us use a black background, you may need to change this to some other color besides basic black to keep your black ink cartridge from going dry in a hurry.

The PostalCAD Settings menu contains a utility for toggling the background color from black to white.  Look for the "Screen Black/white" listing or simply type TBC and hit Enter to toggle back and forth.