How to add hover overlay effect on an image

On our website's page:

http://demo.e3ngage.com/dreams/

You'll see a number of images that look like buttons and are linked to subpages. If you go to the homepage youll see similar buttons but these have a hover effect that was added by an outsourced developer.

Could you please suggest some CSS code to apply to the images on the Our Dreams page to create a similar hover effect as the homepage buttons?

Thanks very much!

Kindly,

AD