gistfile1.html

<mt:Var name="regex" value='/(.*?<img[^>]*src=")([^"]*)(.*)|.*/s' />

<mt:Unless regex_replace="$regex","$2">
<h1>test</h1>
<img src="http://www.example.com/test1.jpg" />
<p>message</p>
</mt:Unless>

<mt:Unless regex_replace="$regex","$2">
<h1>test</h1>
<img src="http://www.example.com/test1.jpg" />
<img src="http://www.example.com/test2.jpg" />
<p>message</p>
</mt:Unless>

<mt:Unless regex_replace="$regex","$2">
<h1>test</h1>
<p>message</p>
</mt:Unless>