Coiso - 2013-06-12

Hello.
On the Greenstone homepage I'd like to know how to change the image of each individual collection instead of having one image for them all.

The page currently looks like the following screenshot, what i'd like to change is the images for each outlined collection. http://img197.imageshack.us/img197/5674/imageyns.png

Looking forward for an answer, thanks.