To set a background color of a drawing panel, use the following call:
Read the next line without the < >
The panel name should be whatever you decided to call your panel in your program and the color is up to you to change.
If you would like to display an image as a background, then I would display the image first as anything else you draw will be on that of that image.
Perhaps there is another way, but this is what I can think of right now.