var images = new Array("images/sites/boevee.png", "images/sites/boevee2.jpg", "images/sites/easytaxvenlo.png", "images/sites/help_louise.png", "images/sites/test.png");
var id = 1;
swap = function(to)
{
   if (to == 'next' && images[(id+2)] != null)
      id = id + 1;
      
   else if (to == 'back' && images[(id-2)] != null)
      id = id - 1;
   
   document.getElementById ("img_1").src = images[(id - 1)];
   document.getElementById ("img_2").src = images[id];
   document.getElementById ("img_3").src = images[(id + 1)];
}