The basic function to use is this:
Grid_DrawImage("background", GetFileURL("bg.jpg"), -25, -25, 50, 50)
The image needs to be in the same folder as your game. The general parameters are:
Grid_DrawImage(id, url, x, y, width, height)
The starting location of the grid is 0,0. I don't know how the image scales. With the values I had above and a fairly large image, it expanded well beyond the limits of the view (which is good since the grid/map scrolls). I'd say just play around with it.
I don't know why this function isn't documented...