I updated the album art on Ampache, but Viridian didn't update.

This is because Viridan caches the album art when it first encounters a new album.

There are 3 simple ways to update the cache.

You can go to File -> Clear Album Art to clear all of the cached album art in Viridian.
Right-click the album art and hit "refresh album art", which should force it to redownload the artwork.
Or just click the album art, that should trigger a refresh.