You could first load the image, and then, when it has finished loading, set it as background-image. That way the browser will (hopefully) load the background image from the cache instead of redownloading it. As you requested it as a plugin:
$.fn.smartBackgroundImage = function(url){
var t = this;
//create an img so the browser will download the image:
$('<img />')
.attr('src', url)
.load(function(){ //attach onload to set background-image
t.each(function(){
$(this).css('backgroundImage', 'url('+url+')' );
});
});
return this;
}
Use it like this:
$('body').smartBackgroundImage('http://example.com/image.png');