To change the background:

• Make a jpg (or jpeg) image using photoshop or paint, or any other graphics program.
• When you save the image, save it to the videoGameDev/learn3D/scenes folder (that's the folder that you've been putting most of your files in), and make sure that it is of format jpg, or jpeg
• Save it as background.jpg
• When you run the game, it should be the new background