For the O2 theme (and perhaps others), make the wallpaper to be 240 x 260 (width x height) with a 75 pixel whitespace area at the top.
The way I do it is to make an image 240 x 185 and then set the canvas level to 240 x 260, moving the actual image to the bottom. Its quite simple. The reason for the whitespace is to allow for your image to fit exactly as it should. The viewable canvas for a background with this theme is 185 pixels with a 75 pixel header space (covering the entire 260 vertical pixels).
__________________ In the beginning the Universe was created. This has made a lot of people very angry and is widely regarded as a bad move.
240x260 includes both the title bar and the ribbon;
240x195 excludes the title bar but includes the ribbon;
240x185 excludes both the title bar and the ribbon