NS4.5 findet innerhalb <div> tag imgage nicht ! warum??

[Armin]

function wechsel()
{ b1 = "img/gif/b.gif";
  document.c.src=b1; }

<div id="cb" class="cbut">
<a href="#" onmouseup="zu()" onmouseout="wechsel2()" onmouseover="wechsel()">
<img src="img/gif/c.gif" width="11" height="11" border="0" align="right" name="c"></a></div>