Q1490 How do I only enable a submit image button when one or more checkboxes within a form have changed?

<form name="myForm">
<input type="checkbox" onClick="if (document.images) document.images['subbut'].src='enabled.gif'"> - Check
</form>

<a href="javascript:;" onClick="if (document.images && document.images['subbut'].src.indexOf('disabled') ==-1) document.myForm.submit(); return false"><img name="subbut" src="disabled.gif"></a>