Logos are used for your blog branding. It is suggested to add logos for your Typlog blog. There are four type of logos in the settings.
image/vnd.microsoft.iconformat image, used for browser tabs.
A png image for iOS and Android. Better in 180x180 size.
The logo used on your blog home page. Better in 256x256 size.
The publisher logo for AMP articles. Better with a height of 60px.
Here is a guide on publisher logo.