The documentation seems to live here: https://help.shopify.com/en/manual/apps/apps-by-shopify/shop-app
The store's profile includes the store's logo, name, and description. You can customize the following details in the Shop profile for your store:
I hope they update it to include information about the Cover photo and social links
Thanks, Austin! That did it.
Unfortunately, that's an extremely short-sighted, lazy solution for Shopify. We use a white PNG of our company name - with alpha channel - against a dark background in our checkout. Since the app's icon has a white bg, our avatar looked as if it were blank. Also, even IF the image were visible, our entire company name crammed into a tiny circular avatar would've looked like a thin black line. Why on Earth would Shopify assume the same file you use in checkout would naturally work for that freaking avatar?!? Par for the course for Shopify, I guess. They always launch things with tons of loose ends, rather than completing the thought. SO frustrating!
Anyway... I hard-coded the logo in our checkout, then made a smaller icon to upload to our checkout settings so it could be used for the app's avatar. Seems to work fine. Thanks for the heads up, and sorry to dump my frustrations on you. I've just wasted so much time trying to blindly figure out Shopify's loose ends. Ugh...