docs: add u-photo for images

for #48, https://github.com/snarfed/bridgy-fed/issues/33#issuecomment-440395450

static/index.html

@@ -50,6 +50,7 @@ and the rest of the <a href="https://en.wikipedia.org/wiki/Fediverse">fediverse<
 <li><a href="#sites">Which sites are supported?</a></li>
 <li><a href="#setup">How do I set it up?</a></li>
 <li><a href="#use">How do I use it?</a></li>
+<li><a href="#image">How do I include an image in a post?</a></li>
 <li><a href="#discovery">How can people on the fediverse find me?</a></li>
 <li><a href="#troubleshooting">I tried it, and it didn't work!</a></li>
 <li><a href="#cost">How much does it cost?</a></li>
@@ -206,6 +207,18 @@ The webmention source URL will usually be a proxy page on <code>fed.brid.gy</cod
 </p>
 </li>
 
+<li id="image" class="question">How do I include an image in a post?</li>
+<li class="answer">
+<p>
+Use <code>&lt;img class="u-photo"&gt;</code> for the image in your post. For example:
+
+<pre>
+&lt;img class="<span class='keyword'>u-photo</span>" src="<span class='value'>/full_glass.jpg</span>" /&gt;
+I love scotch. Scotchy scotchy scotch.
+</pre>
+</p>
+</li>
+
 <li id="discovery" class="question">How can people on the fediverse find me?</li>
 <li class="answer">
 <p>In general, all you have to do is use Bridgy Fed to interact with the fediverse once. Send an original post from your site, like or repost something, follow someone, etc. Then, when other users search for <code>@yourdomain.com@yourdomain.com</code>, they should find your profile!