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However, in your case I assume you want to be 100% sure that the background loads last. You could eliminate a regular body background declaration altogether - then add some javascript onLoad that declares a background image. However people with js turned off would get NO background at all ever.
In case you aren't concerned about non-js visitors, this might be one direction to try:
In the head:
<script type="text/javascript">
bgimg = new Image();
bgimg.src = "[url for your background image]";
</script>In the body tag:
<body onload="document.body.background=bgimg.src;">
Using the same pre-loader script in the head, but a normal background declaration in the body, would pre-load the background-image and help it to display earlier in the rendering cycle.