$.preloadImages = function()
{
  for(var i = 0; i<arguments.length; i++)
  {
    $("<img>").attr("src", arguments[i]);
  }
}

        $.preloadImages("style/nav/home_g.png",
                        "style/nav/therapists_g.png",
                        "style/nav/services_g.png",
                        "style/nav/visit_g.png",
                        "style/nav/testimonials_g.png"); 
$(
	function()
	{
		// set up rollover
		$("img.rollover").hover(
			function()
			{
				this.src = this.src.replace("_d","_g");
			},
			function()
			{
				this.src = this.src.replace("_g","_d");
			}
		);
	}
)