if (document.images) {

kontaktup       = new Image();
kontaktup.src   = "images/kontakt_0.png" ;
kontaktdown     = new Image() ;
kontaktdown.src = "images/kontakt_1.png" ;
impressumup       = new Image();
impressumup.src   = "images/impressum_0.png" ;
impressumdown     = new Image() ;
impressumdown.src = "images/impressum_1.png" ;
startseiteup       = new Image();
startseiteup.src   = "images/startseite_0.png" ;
startseitedown     = new Image() ;
startseitedown.src = "images/startseite_1.png" ;

}
function buttondown( buttonname )
{
    if (document.images) {
      document[ buttonname ].src = eval( buttonname + "down.src" );
    }
}
function buttonup ( buttonname )
{
    if (document.images) {
      document[ buttonname ].src = eval( buttonname + "up.src" );
    }
}