As far as I've been able determine there is no X-Y offset available for HDBackgroundImageUrl & SDBackgroundImageUrl, it's always locked to the upper right-hand corner of the screen. The only place I ever saw BackgroundImageUrl documented was in the Roku Developer Blog, I can't find anything about it in the 4.8 developer wiki.
One way to "move" the image is to start with a 1280x720 or 720x480 png with a fully transparent background and then paste the image that you want to see on the Roku screen at the corresponding location within the transparent png.
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