Hey there,
Here's how to properly size and SEO optimize your product images:
If you're using product images in a lightbox (like for gallery images), the width should be 1500 px, with a resolution of 72 px/in.
If you're using in-line images, the width should be 750 px, with a resolution of 72 px/in.
You can resize images on a Mac by opening your image in Preview > Tools > Adjust Size. Then type in the width dimension (1500px or 750px), and change the resolution to 72 pixels/inch. Make sure the box that says scale proportionally is checked. Then download the image as a JPEG.
You can further optimize your images by compressing the image files with tools like https://tiny-img.com/ or https://tinypng.com. Then upload these to your Squarespace gallery.
Hope that helps 😉