A background is nothing more than an image created in bmp, png, or jpg format. I recommend png as the best format for Prontos. The full iPronto screen is 630x470 pixels. (Could be a couple of pixels less in each dimension). If you want a background that covers the screen you need to make your background graphic or photo that size using a graphics program like Photoshop or Paint Shop Pro. Once you have it the way you want it and sized it correctly, save it as a png file (use the PNG Export command in Paint Shop Pro).

Then create a panel on the screen (Insert/Panel) into which you want the background to appear. Go to Properties/Appearance Tab and import your graphic.