it looks for a folder called "asset_previews" next to the library file (assets.json) currently the name of the asset needs to match the name of the PNG. lowercase and with underscores instead of spaces
This is a Blender 2.8+ port of the Powerlib Addon developed for Agent 327: Operation Barbershop. The Addon was created by Inês Almeida, Francesco Siddi, Olivier Amrein and Dalai Felinto based on work done by Bassam Kurdali for the Tube Project.
Copy or symlink the folder to your Blender user Addon folder.
How to use
The Powerlib tab is located in the 3D View properties panel. Create an empty assets.json file in your project repository (this way you can also version control it).
Select your json file in the powelib settings.