var lastpicture = 1;

function picswitch(picture,site,getdirname) {
  document.getElementById('bild').style.backgroundImage = 'url('+getdirname+'/images/'+site+'_'+picture+'.jpg)';
  document.getElementById('button_'+lastpicture+'').className='button_off';
  document.getElementById('button_'+picture+'').className='button_on';
  lastpicture = picture;
  return false;
}