Remove image helper from slider component

Image helper is doing crazy things, adding all classes (`is-portrait`, `is-squarish`, `is-landscape`) to unloaded images.

![screen shot 2014-06-19 at 23 49 29](https://cloud.githubusercontent.com/assets/271645/3334737/9c106142-f804-11e3-9614-a80b2382744c.png)