.img_hover_change{
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
}
.img_hover_change.cover{
    position: relative;
    z-index: 1;
    opacity: 1;
    transition: opacity .9s;
}
.img_hover_change.only_one{
    opacity: 1!important;
    z-index: 0;
}
.thumbnail-container:hover .img_hover_change.cover{
    opacity: 0;
}