It might or might not work without the quotes, depends on the value of $custom3. Is it TRUE or "TRUE"?
Aside from that, there is no reason for the elseif, why are you echoing nothing? And where does the </table> tag ever get echoed if $custom3 isn't TRUE or "TRUE"?
Here's how I'd write it so you don't have to deal with nested quotes and assuming $custom3 is boolean (untested):
if ($custom3 ){
?>
<tr><td><img src="../../image/<?php =$row[16] ?>"></td></tr>
<?php
}
?>
</table>