Change opacity of images on hover

When users hover over modules with images, the image changes opacity, like so:

Normal state:

![screenshot 2014-04-03 20 29 44](https://cloud.githubusercontent.com/assets/109774/2608064/6fcf7efe-bb66-11e3-99e3-cab4f839875a.png)

Hover state:

![screenshot 2014-04-03 20 29 47](https://cloud.githubusercontent.com/assets/109774/2608066/765f5df2-bb66-11e3-8cd7-426cf9b6c86e.png)