Add PromDash screenshot and img formatting.

content/docs/visualization/promdash.md

@@ -11,6 +11,8 @@ PromDash is a browser-based dashboard builder for Prometheus. It is a
 [Rails](http://rubyonrails.org/) application and stores its dashboard metadata
 in a configurable SQL backend.
 
+[![PromDash screenshot](/assets/promdash_event_processor.png)](/assets/promdash_event_processor.png)
+
 For installing and running PromDash, see the
 [README.md](https://github.com/prometheus/promdash/blob/master/README.md) for
 more information. The instructions below assume that you have a running

static/docs.css

@@ -163,6 +163,13 @@ a.sc-logo img {
   visibility: visible;
 }
 
+.doc-content img {
+  width: 90%;
+  margin-left: auto;
+  margin-right: auto;
+  display: block;
+}
+
 pre {
   font-family: "Courier New", Monaco, Menlo, Consolas, monospace;
   background-color: #444;