backgroundIMG.pspr is a background image - the main header image. It is a .jpg file that is just renamed into .pspr for convinience. Image dimensions are:
width: 480px, height: 80px.
The top area of the image is always visible (except for the full screen mode) and is used as a background for title information. The area allowed for that should be 480px wide and 15px tall. When PSP Radio is in shrunk mode, the button bar would move right under this title area of the image.
The dynamic stream information could be placed over the backgroundIMG.pspr by the means of a transparent 'div'. However, if song and bitrate information can go anywhere, the station info should go over the title area of the image as it stays visible in shrunk mode.
This is the example of a sample backgroundIMG.pspr:
Those dark fields are reserved for song and bitrate information and are later covered with information 'divs' that have transparent backgrounds.
So it is up to you how to design your background and where to place information 'divs', just make sure you can distinguish their content from background.