var theImages = new Array();

theImages[0]  = 'immagini/img_header.gif';
theImages[1]  = 'immagini/img_col_sx_titolo.gif';
theImages[2]  = 'immagini/img_col_dx_titolo.gif';
theImages[3]  = 'img_menu_sx_sfondo.gif';
theImages[4]  = 'img_menu_dx_sfondo.gif';

var i = 0;
var p = theImages.length;
var preBuffer = new Array();
for (i = 0; i < p; i++) {
  preBuffer[i] = new Image();
  preBuffer[i].src = theImages[i];
}