Product images you upload will be automatically resized to 800px wide with height proportional to your original image.
However, there's currently a minor bug in resizing "portrait" oriented images (taller than they are wide) that can make them appear a bit fuzzy, so to avoid that bug if your original image has that orientation, you may wish to edit your image to make it at least slightly wider than it is tall -- e.g., by either cropping it or adding some blank side margins -- then upload that version of the image instead.
