I think that it is not the backround that youhave to change. You could change the opacity when the kensor is hover an image eg
.overlay { background:rgba(0, 0, 0, .90); opacity:0;
}
.img:hover .overlay { opacity:1;
}
Imagine that the overlay is a glass above your images and it is transparent, and when the mouse is hover it the opacity changes from 0 to 1
try it andtell me.