var pic_on   = new Array(1);
var pic_off = new Array(1);
var i;

for(i=0 ; i<4 ; i++)
{
  pic_on[i]  = new Image();
  pic_off[i] = new Image();
}

pic_on[0].src  = 'images/start/jetzt_konfigurieren_on.png';
pic_off[0].src = 'images/start/jetzt_konfigurieren_off.png';

function on(fPosIdx,fImgIdx)
{
  document[fPosIdx].src = pic_on[fImgIdx].src;
}

function off(fPosIdx,fImgIdx)
{
  document[fPosIdx].src = pic_off[fImgIdx].src;
}