A background image can use the border system. This allows for a certain section of the image to always be visible, while everything beyond these borders is used to fill the edges of the screen if the screen ratio mismatches the ratio of the image section.
For further details, see here.
500 pixels and a height of 400 pixels.section starts
200 pixels, and a Y-position of 100 pixels.
500x400 pixels will always be visible, and this section starts at the coordinates (200, 100) of the base image.