It actually does use online artwork databases, but the images are downloaded on demand (unless disabled in rp-config).This project was fun to contribute to, but it's dead. rom-properties (https://github.com/GerbilSoft/rom-properties) is much easier to set up and no hassle with box art databases involved.
Databases used:
- GameTDB: Wii, GameCube, DS, 3DS
- amiibo.life: amiibo