I'm a little late to the party, but I just ran into this issue too. I ended up just creating a blog on how to do it, since I had some previous experience with CSS. Hopefully it'll be corrected in a future Squarespace update.
https://codyturk.com/blog/how-to-change-squarespace-video-gallery-thumbnail-images-to-16x9
Edit: Sorry this blog post was removed awhile back. Big thanks for someone emailing me to bring it to my attention.
Try this code. Class name might be different for some.
// Video Thumbnails 16x9
.lessons.collection-content-wrapper .grid-image-wrapper {
padding-bottom: 56.25%;
}