20103 - Handle the situation where iBL is unable to return an image

[SUBMITTED 02/10/2014 @ 13:26]
- Updated `getImage` within the `Elements` model to return a URL to the placeholder image (uploaded in separate PR)
- Updated the `ElementsTest` accordingly

https://jira.dev.bbc.co.uk/browse/IPLAYER-20103
The default values are not the same here and in `getImage`. Are you sure this won't cause issues elsewhere? In fact, these defaults look  more sensible (16:9) than `getImage`'s (~= 9:16)