<script language="javascript">
    var desc = new Array ('Description', 'Second', 'Yahoo!!!');
    function show_desc (id) {
      document.getElementById('description').value = desc[id];
      return true;
    }
  </script>
...
<img src="image_1.gif" onclick="show_desc(0)"><br>
<img src="image_2.gif" onclick="show_desc(1)"><br>
<img src="image_3.gif" onclick="show_desc(2)"><br>
<form><input type="text" id="description" value=""></form>