Probá con algo así:
Código PHP:
<form id="form1" name="form1" method="post" action="">
<table>
<tr>
<td><input type="checkbox" name="lista"></td>
<td><input type="button" name="Submit2" value="desmarcar" onclick="this.parentNode.parentNode.getElementsByTagName('input')[0].checked=0;" /></td>
<td><input type="button" name="Submit" value="marcar" onclick="this.parentNode.parentNode.getElementsByTagName('input')[0].checked=1;" /></td>
</tr>
<tr>
<td><input type="checkbox" name="lista"></td>
<td><input type="button" name="Submit2" value="desmarcar" onclick="this.parentNode.parentNode.getElementsByTagName('input')[0].checked=0;" /></td>
<td><input type="button" name="Submit" value="marcar" onclick="this.parentNode.parentNode.getElementsByTagName('input')[0].checked=1;" /></td>
</tr>
<tr>
<td><input type="checkbox" name="lista"></td>
<td><input type="button" name="Submit2" value="desmarcar" onclick="this.parentNode.parentNode.getElementsByTagName('input')[0].checked=0;" /></td>
<td><input type="button" name="Submit" value="marcar" onclick="this.parentNode.parentNode.getElementsByTagName('input')[0].checked=1;" /></td>
</tr>
</table>
</form>
Por las dudas: los input type image son equivalentes a botones tipo submit, quizás te convenga reemplazarlos por imágenes normales.