You only need to cut off the part of the image that is unique (the top in your case). The rest can be repeated without the human eye noticing it is being repeated.
From the look of your image, I'd say you could cut of 200-300 pixels off the top (with the full width of course) and display in the #wrapper element. The rest of the image is just white (or almost white) background with some dots on it, and could be repeated in the body background.
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