Is simple Upload the image to your file manager. I personally like to keep all the images in a folder named images just to keep things neat.
Then in your HTML document near the beginning is the followinbg line
it might have some additional coding but in this line add the following
Replace URL with the link to your image. And this will give you the background. If you want the background to be a solid color instead using the below.
ffffff is for white but this can be replaced with any color code to select any color you wish. Here's a neat free program that will help you select the color and give you the colors code.
http://www.colorcop.net/