Checkout Invoice Logos
For Letter/A4 prints size doesn't really matter. If a logo looks good when displayed or printed with Preview, it will look good when printed with Checkout. If customers have their logo in a file that's intended for use on a letterhead, that will be perfect for Checkout.
JPG, PNG or PDF are the only good file formats to use for this. GIF will also work, SVG might work.
Thermal receipts are a completely different story. The only logos that will print reasonably well on a thermal receipt are PDF logos, and then only if the logo contains an actual vector drawing and not bitmap images. This is the main reason we don't ship Checkout with a template that will print logos on thermal receipts.
Checkout embeds the image in the database.