Hiding the broken image icon for closed organisations

Users don't need to see an image of the old website. They also don't need to see a broken image icon if there isn't an image.
![some_closed_orgs_have_no_website_screenshots](https://f.cloud.github.com/assets/1897393/2513815/39c93a50-b433-11e3-9962-afd17e64ab24.jpg)