A picture is worth 100 words, but only 10-15 songs approximately. If it’s doing its job correctly, an album cover should capture the essence of its audible counterpart perfectly. It should, at the very least, be the artist’s visual interpretation of the music. At best it should assume an importance that suggests the music might have been written to showcase the image, rather than the picture chosen to represent the music.
I’m not looking for the most iconic here, though if you think that the covers of Sgt. Pepper’s, Nevermind or The Freewheelin’ Bob Dylan really are the ideal representation of their musical equivalent then feel free to include them, but I’d prefer covers by which you really can judge the musical book (provided they aren’t as awful as one another). Things like the intimidated first-person narrative of Straight Outta Compton, the theatrical, caricatured and vaguely sleazy embrace of Rain Dogs or the grotesque beetle adorning Mezzanine, which perfectly symbolises the way the album crawls starkly and sinisterly through its eleven tracks. Or, you know, they could just be really cool. Maybe include a few words to say why the image is suited to the music.

My 5:

Massive Attack – Mezzanine (see above)
The National – Boxer (late night, noiresque, realist)
The Black Keys – Thickfreakness (unsophisticated, thick/viscous riffs)
White Denim – Workout Holiday (fun, collage, homemade aesthetic)
Wild Beasts – Two Dancers (arty, salacious, sleek)