Fanart.tv is also another one with lots of high resolution album covers, artist images, disc art (mostly CDs). They’ve recently updated album covers to 1000x1000. You need to first search the artist name, then click on music option on left to see all the artist related images.
Fanart.tv has most movie related images too. It’s one of the online resources used by Kodi when it auto downloads images for its UI.