function set_conc_pics(){
    if (document.images){
        conc_link_plain  = new Image();
        conc_link_plain.src   = "http://groundtest.co.nz/concrete/conc_cyl_1.jpg" ;
        conc_link_over  = new Image() ;
        conc_link_over.src = "http://groundtest.co.nz/concrete/conc_cyl_break.jpg" ;
    }
}

function button_over(){
    if (document.images) document.images.conc_link.src = conc_link_over.src;
}

function button_out(){
    if (document.images) document.images.conc_link.src = conc_link_plain.src;
}