Resize images to fit box

Currently, image is partially clipped by box's size as shown below. If the image has large white margin area, there can be only white box displayed. 
![image_can_be_displayed_as_blank](https://f.cloud.github.com/assets/2943548/1846213/4e5112ee-75d2-11e3-80e7-73ad904e9d6c.png)

This PR add a single line css for resizing image to fit box's size.