User can add an image to auction

https://trello.com/c/rPefPOmu/13-as-a-user-i-can-add-images-to-posts

![image](https://cloud.githubusercontent.com/assets/8681076/4999535/212e7652-69b1-11e4-80a1-5afa729b4b5f.png)

This is actually OK - when you're not creating a model, you can usually do `Model.new`, but when you're saving it (like in `create`), you'll do `post.models.new` to associate it with a given post before saving.

It's fine as-is.