The ideal format for your business logo is vector. It should be 400 pixels by 400 pixels. If the logo isn't square, it will get distorted when it appears during checkout. You can upload your logo from the Dashboard by going to Edit Business → Business Logo. Unless you select another image, this logo will appear during the checkout process. To use a different image, you'll need to host that image outside of Piano (publishers often upload their chosen image to their production site) and then point to that image using an src attribute.