oldImg = ''; 

function setImage(filename)
{ 
	var elem = document.getElementById('mainImage');
        oldImg = elem.src;
	elem.src = 'fileadmin/uploads/downloads/' + filename;
} 

function unSetImage()
{
      if(oldImg == "") return;
      var elem = document.getElementById('mainImage');
      elem.src = oldImg;
}