So im trying to make my gallery look like the one shown below at
But when I try to make it, the photo that I want doesn't appear behind it and looks like this
So how do I make it so that I can get a photo behind the gallery? I asked an admin over at that fandom and he said use CSS and he gave me a code that looks like this
background-image: url(<image LINK>);
But I still don't understand what to do. Any help would be appreciated. Thanks!