function but_rollover(nummer){
document.getElementById(nummer).src="img/rahmen_top"+nummer+"_over.jpg";
}

function but_back(nummer){
document.getElementById(nummer).src="img/rahmen_top"+nummer+".jpg";
}

function but_rollover_bottom(nummer){
document.getElementById(nummer).src="img/rahmen_bottom"+nummer+"_over.jpg";
}

function but_back_bottom(nummer){
document.getElementById(nummer).src="img/rahmen_bottom"+nummer+".jpg";
}