If you're trying to make the wallpaper full screen, (no top or bottom panel showing), set the top panel to "false". The center panel, or wallpaper will fill in the space for the top panel. The bottom panel is usually set to 100 px by default. So if you are starting with a 1024x768 wallpaper, you would crop 100 px off the bottom of the wallpaper and use it for the bottom panel. The remaining 1024x668 image would be used for the center panel, and with the top panel set to false, it would fill the rest of the screen.
As Bichur said, it is kind of trial and error to get it exact, but that's the basic idea of how you do it.