Rollover on image

Hello,

My site: https://www.12field.com.au

Half way down the home page, under Projects, I would like to have a rollover on each thumbnail. So, when the mouse moves over the thumbnail image (quote on wood) this images appears on top at 80% opacity:

https://www.12field.com.au/wp-content/uploads/2013/09/rollover.png

(a black thumbnail, same size, with a cross)

Alternatively, if placing the black rollver ON TOP of the image is too tricky, I would be happy to settle for an image swap. How would I place that in my wp page?

I’d love any help, thanks heaps!