There are some album covers that are just iconic (Nirvana's 'Nevermind' and U2's 'Boy' are two that come to mind). Because the album art is the first part of the album to reach consumers, and is the visual representation of the band, artists are usually picky about what graces the cover of their album. Many times it's a picture of the musicians themselves that is featured on the cover, or a piece of artwork.
But sometimes the band chooses a photograph that represents their sound or their image indirectly. They stage photo shoots with models, snap photos of people on the street, or choose from an old portfolio of photographs.