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

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

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

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

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