<form id="form1" name="form1" method="post" action="">
  <p>请输入图片地址
    <input type="text" name="a" id="a" />
     
    <input type="button" name="button" id="button" value="输入" onclick="g()">
  </p>
  <p>
    <textarea name="b" id="b" cols="45" rows="5"></textarea>
</p>
</form>

<script language="javascript">
 function g()
 {
   var str = document.getElementById('a').value
   str="<img>"+str+"</img>"
   document.getElementById('b').value=str
 }
</script>