<%= image_tag("header/home_but.gif", :mouseover => "header/home_over.gif") %>

Prototype.preloadImages = function(){
    for(var i=0, images=[]; src=arguments[i]; i++){
    	images.push(new Image());
    	images.last().src = src;
    }
};

Event.observe(window, 'load', function(){
    Prototype.preloadImages('header/home_over.gif','another/image/to/preload.gif');
});