IMO one of the main considerations when it comes to an album cover is
Branding
Whatever image or piece of art that is chosen as the album cover will be forever associated with the music and the ideas and vibes that the musicians were trying to get across at the time. Whether the listener really understands how the album cover ties in with the music or not, if there even is such a connection, in the end the album cover ends up representing the album and the music on it from a branding standpoint. So IMO it makes sense for the musicians to pick something that
makes sense to them, since they are the ones who presumably were the creative forces behind the writing of the music and lyrics.. so they might as well get to decide what image will represent the music and the ideas the musicians were trying to get across on the album.
Branding can serve multiple purposes.. It can be functional, in that it can tell the audience what kind of music to expect. If the album cover has trumpets on it, the listener would probably expect to hear trumpet sounds in many of the songs. If it's a bunch of mean looking guys with long hair who are wearing leather and standing over the carcass of a dead animal, the message being sent is that the music might appeal to those who enjoy specific subgenres of metal. Those who are a part of that subculture would be able to look at that album cover and understand that this is the music of their people. Those who prefer to listen to polka music and dislike black metal would be able to look at that album cover and very quickly decide to move on to something else. So, I see many album covers serving functional purposes, from a branding standpoint. They are an attempt to drive the right kind of people to the music, warning others to stay away.
Sometimes I feel that the album cover is more about the musician rather than the specific album at hand.
Some album covers attempt to tie hopefully iconic imagery to the music. If the music is iconic too then that can end up working quite well.
In the end this is.. well, art. So there are many reasons and justifications for why someone used this and that as an album cover, and the audience can have their own opinions and points of view on this as well.. The same album cover can mean different things to different people. But in the end album covers remind us of the music that they are associated with, whatever interpretations of them we might all have in our minds or what interpretations of them the original artists might have had when they picked them.