function RolloverImg(ruta,imgName){
document.images['imgId'].src = ruta+imgName;
}

var i; 
var imagenes = new Array("img_hdr01.jpg","img_hdr02.jpg","img_hdr03.jpg","img_hdr04.jpg","img_hdr05.jpg","img_hdr06.jpg","img_hdr07.jpg"); 
var lista_imagenes = new Array(); 

function preload_images(ruta){ 

for(i in imagenes){ 
lista_imagenes[i] = new Image(); 
lista_imagenes[i].src = ruta+imagenes[i]; 

} 

}