It's nice and easy to embed images in markdown files on Github.
![alt text](http://mysite.com/upskirt.png "Title Text")
But for external assets Github will serve these via Akamai, and the reference seems to only get updated when you push changes to the markdown file.
So for example (which was my case) I was linking to an app dependencies image (http://clj-deps.herokuapp.com - like Gemnasium), and my application continued to show as being unstable when it wasn't. I pushed some changes to the README and it updated!
This info is only from my experience of this behaviour, so I'm not exactly sure if/how the expiration is set up. But something to watch out for if you're getting some odd behaviour.
Update: Travis CI images seem unaffected.