Avoid shrinking logo

Bootstrap sets both the max-width and max-height of all image elements to 100%.  When attributions are removed from their containing element, this causes logos to shrink.

This can be reproduced by opening the [vector layer example](http://openlayers.org/en/master/examples/vector-layer.html) and running the following:
```js
map.getLayers().removeAt(0)
```

![tiny](https://cloud.githubusercontent.com/assets/41094/4782019/8d411416-5cd3-11e4-950b-6f0597646941.png)