var currentThumbId = 0;

function changeImg(changewhat, changeborder, changeto) {
  var changethis = document.getElementById(changewhat);
  changethis.src = changeto;

  if( currentThumbId ) { 
    var oldborder = document.getElementById(currentThumbId);
    oldborder.style.borderColor = 'black';
  }

  var newborder = document.getElementById(changeborder);
  newborder.style.borderColor = 'green';
  currentThumbId = changeborder;

  return false;

}