Text on image when hovering, how?

Hi there!

I want the text of a widget to be in the image when you hover over the image. At the moment it looks like this: https://www.lennardfopma.nl. Scroll down and you see four images and text below them. I did the transparancy-change on the images -so glad that worked out -, now the text. How do I fix that and which css code do I need to use? Hope you can help me out!