$(document).ready(function() {
	var regex = /\.(jpg|png|gif)/i;
	var replacement = '-over.$1';
	
	var image_set = $('#header img.rollover');
	
	// Preload images
	image_set.preload({
		 find: regex
		,replace: replacement
	});
	
	// Do rollover
	image_set.hover(function(){
		this.src = this.src.replace(regex,replacement);	
	},function(){
		this.src = this.src.replace(/-over\.(jpg|png|gif)/i,'.$1');
	});
});