This example presents the fastest way to load a background image into Processing. To load an image as the background, it must be the same width and height as the program. 

y = 0

def setup():
    size(640, 360)
    # The background image must be the same size as the parameters
    # into the size() method. In this program, the size of the image
    # is 640 x 360 pixels.
    global bg
    bg = loadImage("moonwalk.jpg")


def draw():
    global y

    background(bg)
    stroke(226, 204, 0)
    line(0, y, width, y)
    y += 1
    if y > height:
        y = 0