Add an Image that Overlays on Hover over Another Image

Hi there,

On the website, if you scroll to the bottom of the homepage, you will see three circular images that serve as button links to sub pages. We would like to have a grey circular image over top of the one currently there. When the user hovers over the grey circle, the image that appears is the one that is currently there. Could you please provide the CSS to do that? I guess I may have to switch the current images out with the grey ones? Please let me know how best to proceed.

Thanks and regards,