Twentyfifteen has a feature to add a custom image as a background to your sidebar. While this could be used to implement a logo or branding of some sort, it gets complicated when the sidebar switches to become the header on smaller screens. This is only one of two “enhancements” I made to Twentyfifteen. I really felt it needed a solid space dedicated to a logo of some sort and I’m surprised it wasn’t included in the first place.
Where to find it?
Customizer > Header Image > “Header Logo”
The Upload Custom Logo feature is exactly what you expect: It allows you to add a logo to your “header”. This fits automatically within the “header” whether you are in large screen-sidebar view, or small screen-header view.
I’ve added another feature called “Header Text Positioning” which will allow you to position the site title and tagline in relation to the logo. See that page for details.