Users can add and delete up to three images

https://trello.com/c/eWIDTDGe/21-as-a-user-images-i-add-to-my-existing-listing-can-be-deleted-and-wont-be-duplicated

![image](https://cloud.githubusercontent.com/assets/8646429/5000930/b1fe447a-69be-11e4-88af-c5c048e5250b.png)

How about extracting this to a constant on Listing, like `Listing::NUMBER_OF_IMAGES`?