var nr = 1;
var theMax=20;

function doNext(){
  if(nr>=theMax) nr = 1;
    else nr++;
  if(nr<10) nnr="0"+nr;
    else nnr = nr;
  document.getElementById("photos").src="images/photogallery/photo-"+nnr+".jpg";
}

function doPrev(){
  if(nr<=1) nr=theMax;
    else nr--;
  if(nr<10) nnr="0"+nr;
    else nnr = nr;
  document.getElementById("photos").src="images/photogallery/photo-"+nnr+".jpg";
}