If you want to make a background image overlay appears when hover images on Gallery Section (all images will use same overlay), you can use this code to Website > Website Tools > Custom CSS.
figure[class*="gallery-"] a:after {
content: "";
background-image: url(https://cdn.pixabay.com/photo/2018/06/22/08/24/emotions-3490223_1280.jpg);
background-size: cover;
background-repeat: no-repeat;
display: block;
position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
opacity: 0;
transition: opacity ease 200ms !important
}
figure[class*="gallery-"]:hover a:after {
opacity: 0.5;
}
Replace Pixabay with your overlay image url.