function zpetb(vfile){
if (vfile==1) 
    {
     document["zpetbo"].src="Images/zpetb1.gif"
     }
if (vfile==2) 
    {
     document["zpetbo"].src="Images/zpetb2.gif"
     }
}

document.write('<a href="main.htm" onMouseOut="zpetb(1)" onMouseOver="zpetb(2)"><img name="zpetbo" src="images/zpetb1.gif" border=0 ></a>')