Checkout: Add your shop logo
To add your shop's logo on the Checkout page, you can choose a logo in the Theme Editor.
Result on Checkout page:

How to:
Open the Theme Editor. For a draft theme, use the Customize link for that theme:

If you're using a live/published theme:

Under Theme settings (1), Checkout (2):

Use the Select an image option and browse to an image in your Shopify library:
Once you've added the logo, there are Position and Height settings below you can adjust:

Save your changes.