Background images

Hey, I am on brave nightly, and I was looking to tinker with something but the github project is bigger than I realized, hopefully someone can point me in the right direction.

I want to know where this section of code is to point to this “repository/library”

and where to add a tab too. Anyone familiar with the repo enough to point me to the section or what to look for? I wanna try to add a custom repo of images to choose from.