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).
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