Hi there,
you have this CSS, probably in Customizer > Additional CSS:
.woocommerce div.product div.images .flex-control-thumbs li {
padding: 5px 5px 0 0;
-webkit-box-sizing: border-box;
box-sizing: border-box
}
@media (min-width:460px) {
.woocommerce-page div.product div.images {
display: -webkit-box;
display: -ms-flexbox;
display: flex;
-ms-flex-item-align: center;
align-self: center
}
.woocommerce div.product div.images .flex-control-thumbs {
-webkit-box-ordinal-group: 0;
-ms-flex-order: -1;
order: -1;
display: -webkit-box;
display: -ms-flexbox;
display: flex;
-webkit-box-orient: vertical;
-webkit-box-direction: normal;
-ms-flex-direction: column;
flex-direction: column;
-webkit-box-flex: 1;
-ms-flex: 1 0 80px;
flex: 1 0 80px;
max-height: 340px;
overflow: auto
}
.woocommerce div.product div.images .flex-control-thumbs li {
width: 100%;
padding: 0 10px 10px 0
}
}
.woocommerce div.product div.images .flex-control-thumbs li {
padding: 5px 5px 0 0;
-webkit-box-sizing: border-box;
box-sizing: border-box;
-webkit-box-flex: 1;
-ms-flex: 1 0 25%;
flex: 1 0 25%
}
.woocommerce div.product div.images .flex-control-thumbs {
display: -webkit-box;
display: -ms-flexbox;
display: flex;
overflow: auto
}
Remove that and they will default to the normal row under the image