The easiest way to do it is to run it in a window. If your monitor is 1024x768, specify the Width and Height in jukebox.ini as, say, 1000 and 744 and you'll jet a jukebox that runs in a window. Performance may suffer this way, though; I only use windowed modes for testing, it's possible that for extended use you might see hiccups in the music.
The next way to do it is to resize the background image. Pull the background in to an image editor and select the option that adds pixels to the page rather than stretching the image. So if you take the image up 50 pixels in both directions, you have a 25 pixel border around the image. Now, though, all the interface elements are off. So you need to edit the skin file and change any numbers that refer to a position, adding 25 to each of them. Don't change numbers that specify widths or heights.
--Chris