function showSmall(){
 document.getElementById('large_cover').style.display = "none";
}

function showLarge(){

document.getElementById('large_cover').style.display = "block";

}

if(document.getElementById("small_cover")){

document.getElementById("small_cover").onmouseover=showLarge;
document.getElementById("large_cover").onmouseover=showLarge;
document.getElementById("large_cover").onmouseout=showSmall;

}