var adv1_off = new Image();
var adv1_over = new Image();

adv1_off.src = "https://www.onefile.co.uk/images/menu_plus.gif";
adv1_over.src = "https://www.onefile.co.uk/images/menu_minus.gif";

function imgact(imgName) 
{
  if(document.images) document.getElementById(imgName).src = eval(imgName + "_over.src");
}

function imginact(imgName) 
{
  if(document.images) document.getElementById(imgName).src = eval(imgName + "_off.src");
}