CSS for adding an image background on the "cards" in image blocks

Hi there,

I'm wondering if anyone could help me with the CSS I would need to add in order to make all "image cards" sitewide carry a background image themselves?

I know next to nothing about CSS, so I'm a bit stumped. I normally just google, copy and paste for anything I need, but can't seem to find anything on this.

Thanks in advance!!

Can you share link to page where you use an image block? We can test some code easier