SOURCE

my$(id) {
  return document.getElementById(id);
}
my$('node_small').onmouseover = function() {
  my$('er').className = 'show';
}
my$('node_small')

<div class="nodeSmall" id="node_small">
  <div class="erweima hide" id="er">
    <img src="" alt="" />
  </div>
</div>

.nodeSmall {
  width: 50px;
  height: 50px;
  background: url() no-repeat -159px -51px;
  position: fixed;
  right: 10px;
  top: 40%;
}
.erweima {
  position :absolute;
  top: 0;
  left: -150px;
}
.nodeSmall a {
  display: block;
  width: 50px;;
  height: 50px;
}
.hide {
  display: none;
}
.show {
  display: block;
}