If they make it part of their art, it's important.
Your reaction to an album packaged in a bubblegum pink sleeve with a pun for a title is likely going to be miles away from an album with a sombre, sepia landscape on the front. Your image is a choice, and every single choice you make gives your music context. It's the difference between Duchamp's "Fountain" and the urinal down at the local tattoo parlour. The title, the context - even the plinth will colour (or are meant to colour) the viewer's reaction to it. Same applies to music. It's the difference between a Weezer album and a Godspeed You! Black Emperor album. It's all part of the finished piece, right down to the gratuitous exclamation mark.
You could argue, if you were being really pretentious (and it'd appear I am - how about that!) that not making image a part of a musician's art can, in itself, be considered an artistic choice. Play music in a t-shirt and jeans and you're saying, automatically, whether you want to or not, "I'm more down-to-earth than that tosser dressed up as a flower." So in that way, even the choice not to make any special choice betrays the artist's mindset, and puts the music into a completely different context. Image shouldn't be important, but I... honestly don't know if it can really help itself. We make stories out of shapes, and we, as a species, are never going to be able to stop.
But that is where this post's going to stop, because if I cram my head any further up my arse I'll risk vomiting my teeth up. Do stop me if I stumble across a sensible point - if I have, I haven't noticed.