Fix CSS for teaser view mode for image gallery.

#6408 

Before:
![selection_422](https://cloud.githubusercontent.com/assets/4497748/3717989/a4930e84-162f-11e4-844a-fccab2761ca8.png)

After:
![selection_421](https://cloud.githubusercontent.com/assets/4497748/3717986/9e85f894-162f-11e4-913a-95647dfe2bab.png)