>>> image
![](http://foo.com/foo.png)

<<<
<p><img src="http://foo.com/foo.png" alt="" /></p>
>>> alternate text
![alternate text](http://foo.com/foo.png)

<<<
<p><img src="http://foo.com/foo.png" alt="alternate text" /></p>
>>> title
![](http://foo.com/foo.png "optional title")

<<<
<p><img src="http://foo.com/foo.png" alt="" title="optional title" /></p>
>>> invalid alt text
![`alt`](http://foo.com/foo.png)

<<<
<p><img src="http://foo.com/foo.png" alt="alt" /></p>