Netscape only allows the onclick even on links, buttons and layers. The best way to do what you want for both Netscape and IE would be to place the image in an anchor tag with the href set to the javascript, such as:
<td width="100"><a href="javascript:window.document.Form21.submit()">
<img src="../assets/images/available.gif" hspace=5 border=0></a>
</td>