Changing the shape of images and product images

You can target the images with CSS. For your home page product images, add this to Design > Custom CSS to make them circular. The collection specifies the page and then the code specifies images in the product block.

#collection-5ef925025708dc5cb9e01f4b .product-block .image-container img {
    border-radius: 50%;
}

For other images, you can specify the block (I use the first plugin in this post: https://christyprice.com/squarespace-tips-tricks/chrome-extensions-squarespace-designers) and then set the border radius.