Document new onwardimage iframe parameter

about/index.html

@@ -41,6 +41,7 @@ <h3 id="embed">Can I add Widen My Path to our own site?</h3>
 				<li><tt>category=cycleways</tt> / <tt>category=track</tt> / <tt>category=closure</tt> will filter the map to the specific type.</li>
 				<li><tt>properties=false</tt> will disable the header bar containing links to our other sites.</li>
 				<li><tt>onward=https://mywebsite.example.com/contactmycouncillor</tt> will provide an additional 'Contact my local councillor' box after suggestion submission, with the given link on your website as the onward address.</li>
+				<li><tt>onwardimage=https://mywebsite.example.com/myimage.png</tt> will add the specified image to the 'Contact my local councillor' box after suggestion submission; the image will be shown as-is, and should be maximum 185px width.</li>
 				<li><tt>twitter=https://mywebsite.example.com/path/to/embedpage</tt> will set the URL on your website to be set in the tweet from the tweet button that ppears after submission submission.</li>
 				<li>A map hash like <tt>#14/52.2009/0.1305</tt> , added at the end, will adjust the map position; the numbers are: zoom level / latitude / longitude.</li>
 			</ul>