Hover effect on image

Hello -

I have been asked to try to bring a hover effect to a GP-themed site. I know GP (or GB) doesn't do this out of the box, but I was hoping someone could point me to a direction on how to do it.

The best way to describe the effect is to actually see it. Go to https://www.selkersearch.com/clients (this is not a wordpress site) and simply mouse over (hover) on any of the client blocks to see an inner box and some text appear. It looks like they are using a javascript library to do this. I need to know (a) is JS the only way to do this, and (b) Do you know of a JS library that achieves this effect, or even a piece of JS code I can inline on the page for the effect?

Thanks in advance

Jon