@benoitcorda - [minor] Add key protection before retrying image downloads

![Screen Shot 2013-02-28 at 13 19 00](https://f.cloud.github.com/assets/49038/205420/765dab26-81a9-11e2-832b-2bb89f6553cc.png)

Unsure why we have a null url, but it should just fail elegantly.