What images are chosen as album covers?
For the web interface, MusicCabinet has a setting called "Prefer last.fm artwork". If this is set to "Yes", artwork available at last.fm will be favored and displayed using image links pointing at last.fm. Artwork from last.fm is not downloaded to your server, only referenced by URL. This is useful if you have bandwidth limitations for your server, as it then no longer needs to serve both images and music to clients. This is only used in the web interface, not for apps (using an external image host is not supported in the official API). That means that some albums might have artwork in the web interface, while not for apps when using this setting.
If last.fm artwork is not used, MusicCabinet uses embedded artwork. If different songs from the same songs have different artwork, the choice of which to use is arbitrary.
If embedded artwork is not found in any song, folder artwork from any folder containing songs from the album is used. Images are picked by name: folder.jpg/jpeg/png/gif first, then cover.jpg/jpeg/png/gif. If different artwork is found in different folders for the same album, the choice of which to use is again arbitrary.