.my-class1:not(.my-class1-2)
document.body.children[0].children[0].style.backgroundImage = document.body.children[0].children[1].children[0].style.backgroundImage;
<html>
<style>
.bigimage {
width:100%;
height:75%;
margin-bottom: 20px;
background-repeat: no-repeat;
background-position: center;
background-size: contain;
}
.thumbnail {
float:left;
width:19%;
height:20%;
border-style: solid;
border-width: 1px;
border-color: #333390;
background-repeat: no-repeat;
background-position: center;
background-size: contain;
}
</style>
<script>
window.onload = function () {
document.body.children[0].children[0].style.backgroundImage = document.body.children[0].children[1].children[0].style.backgroundImage;
}
function img(el) {
el.parentElement.parentElement.children[0].style.backgroundImage = el.style.backgroundImage;
}
</script>
<body>
<div>
<div class="bigimage"></div>
<div>
<div onclick="img(this)" class="thumbnail" style="background-image:url(https://hdqwalls.com/download/willa-holland-5k-2018-tc-1920x1080.jpg)"></div>
<div onclick="img(this)" class="thumbnail" style="background-image:url(https://img3.goodfon.ru/original/1920x1080/6/a4/krasivaya-devushka-amber-heard.jpg)"></div>
<div onclick="img(this)" class="thumbnail" style="background-image:url(https://img1.goodfon.com/original/1920x1080/3/9f/anastasiya-scheglova-devushka-4354.jpg)"></div>
<div onclick="img(this)" class="thumbnail" style="background-image:url(https://img3.goodfon.ru/wallpaper/original/a/86/kira-kristina-naytli-keira.jpg)"></div>
<div onclick="img(this)" class="thumbnail" style="background-image:url(https://wallbox.ru/wallpapers/main2/201731/1501485774597edace3af7c6.85452886.jpg)"></div>
</div>
</div>
</body>
</html>
<html>
<script>
function img(el) {
el.style.backgroundImage = "url(https://img3.goodfon.ru/original/1920x1080/6/a4/krasivaya-devushka-amber-heard.jpg)"
}
</script>
<body>
<div onclick='img(this)' style='width:100%;height:100%;margin-bottom: 20px; background-image: url(https://vdudvdud.ru/wa-data/public/shop/products/30/00/30/images/296/296.970.jpg);'></div>
</body>
</html>
div onclick='img(this)'
cell.children[0].value