var HouseOver = new Image(); 
var HouseOut  = new Image(); 
var MailOver  = new Image(); 
var MailOut   = new Image(); 

HouseOver.src = "img/house_d.gif";
HouseOut.src  = "img/house_u.gif";

MailOver.src  = "img/letter_d.gif";
MailOut.src   = "img/letter_u.gif";


function m_over(num)
{
  if (num == 1)
     document.getElementById("ImgHouse").src = HouseOver.src;
  if (num == 2)
     document.getElementById("ImgMail").src = MailOver.src;
}

function m_out(num)
{
  if (num == 1)
     document.getElementById("ImgHouse").src = HouseOut.src;
  if (num == 2)
     document.getElementById("ImgMail").src = MailOut.src;
}