Have you tried adding a check to see if the is a variant image to the template? something like..
Code:
{foreach name=variants from=$variants item=var}
<td width="160">
{if $var.image_url ne ""}
<img src="{$var.image_url}" width="200" height="200">
<br />
{/if}
{foreach from=$var.options item=o}
{$o.option_name}
{/foreach}
Just a thoughy, I haven't checked it.