Fix product grid thumbnails rendering

Add a style for product grid image rendering to render well on multiple resolutions and for all blocks having the same size.

Before:

![capture decran 2014-01-28 a 10 31 09](https://f.cloud.github.com/assets/103900/2017643/1525fe82-87ff-11e3-8002-b729a4dd0fc4.png)

After:

![capture decran 2014-01-28 a 11 58 49](https://f.cloud.github.com/assets/103900/2018267/4a2a9320-880b-11e3-876d-6a7845b60167.png)