Subscriptions template: test for imagefield
If you can use php in the template (?) I would just wrap the cell containing the image in an if statement, something like this:

Your subscriptions on [site:name] notify you of the following changes since the previous digest:&lt;br /&gt;

{{[subs:items:count]#&lt;img src="http://@mydomain@/Newsletter/fine-line.jpg" width="580" height="18"&gt;&lt;br /&gt;
#1 of [subs:items:count] {{[subs:items:index:#0:subs:is-new]?(new):}}{{[subs:items:index:#0:subs:is-updated]?(updated):}}{{[subs:items:index:#0:subs:is-old]?(commented): &lt;b&gt;[subs:items:index:#0:as-node:title]&lt;/b&gt;}}

&lt;table border="0" width="100%" padding="5"&gt;
    &lt;tr&gt;

        &lt;?php if(**code to determine if image exists**): ?&gt;
        &lt;td valign="top"&gt;&lt;img src="[subs:items:index:#0:as-node:field_news_image]" alt="teaser image" width="100" /&gt;&lt;/td&gt;
        &lt;?php endif ?&gt;

        &lt;td style="font-family: Calibri, Arial, Helvetica, sans-serif;" valign="top" align="left"&gt;[subs:items:index:#0:as-node:summary]&lt;/td&gt;
    &lt;/tr&gt;
&lt;/table&gt; 
Read more: [subs:items:index:#0:as-node:url]&lt;br /&gt;
}}


That way if there's no image it just won't print the cell at all and the other cell will take the entire width of the table.

If you can't use php in the template then I'm not sure how you could deal with it except perhaps by merging the two cells like this:

&lt;td style="font-family: Calibri, Arial, Helvetica, sans-serif;" valign="top"&gt;
  &lt;img src="[subs:items:index:#0:as-node:field_news_image]" alt="teaser image" width="100" /&gt;[subs:items:index:#0:as-node:summary]
&lt;/td&gt;