To move slideshow thumbnails to left of image in Squarespace, you can do these.
#1. First, find the ID of the Slideshow.
In my example, we will have:
section[data-section-id="66b558355d5793599c7f117c"]
#2. Use this code to Custom CSS box
@media screen and (min-width:768px) {
section[data-section-id="66b558355d5793599c7f117c"] {
.gallery-slideshow[data-thumbnails="true"] {
display: flex;
flex-direction: row-reverse;
align-items: center;
}
.gallery-slideshow-thumbnails {
width: 30% !important;
}}}