Código PHP:
<html>
<head>
<SCRIPT LANGUAGE="JavaScript">
<!--
image1 = new Image();
image1.src = "unchecked.gif";
image2 = new Image();
image2.src = "checked.gif";
function cambiar(id) {
if ( document.getElementById('checkbox_'+id).src != image1.src ) {
document.getElementById('checkbox_'+id).src = image1.src;
} else {
document.getElementById('checkbox_'+id).src = image2.src;
}
}
-->
</SCRIPT>
</head>
<body>
<a href="javascript:cambiar('1')"><img id="checkbox_1" name="checkbox_1" src="unchecked.gif" width="14" height="16" border="0"></a>
</body>
</html>