Mine

function toggleForm(divToToggle, imgToToggle)
{
  if (Element.visible(divToToggle))
  {
    new Effect.BlindUp(divToToggle, {});
  }
  else
  {
    new Effect.BlindDown(divToToggle, {});
  }
  
  image = document.getElementById(imgToToggle);
  alert(image.src);
  if(image.src == document.root + '/images/common/red_plus.gif')
  {
		image.src = '/images/common/red_minus.gif';
  }
  else
  {
  		image.src = '/images/common/red_plus.gif';
  }
}