function defineMouseEvent( elem ) {
  if ( $( elem ) ) {
    $( elem ).onclick = function() {
      $( "aImage" ).src = imgs[elem];
      return false;
    }
  }
}

function preloadImages() {
  var img = new Image();
  for ( key in imgs ) {
    img.src = imgs[key];
    defineMouseEvent( key );
  }
}