Make preview images better

I actually dislike that the previews are currently shown so little. This offers not really any advantage to the user.

This PR shows the preview a little bit larger:

**Before:**
![screenshot 2014-04-16 18 56 39](https://cloud.githubusercontent.com/assets/878997/2722445/1bc9575e-c588-11e3-8bab-4dc2faf06c02.png)

**After:**
![screenshot 2014-04-16 18 53 04](https://cloud.githubusercontent.com/assets/878997/2722403/a40d6f70-c587-11e3-90b5-02f6bcbf7183.png)

I'm not a CSS nor design guru, feel free to adjust anything in this PR ;-)

@jancborchardt @VicDeo