Most cover art of a metal bands shows the band with scowls in their faces. It's not just the metal bands. I have seen this on a lot of other music promo covers. 
 
 Is there some association with the music that one can't or shouldn't be happy or smile? It's like the photographer said, " Ok now give me the most despicable example of lemon/lime eating you can conjure. If you can't do that at least look as if you have no soul." Look like you hate the world and want to kill it. 
 
 Is the  band regarded as namby pamby if someone cracks a smile? Is this some kind of unwritten rule? 
 
 The whole look kind of hearkens back to a time when photographers never asked or expected anything other than a stoic face. Hey maybe these guys really are that miserable.

Me thinks they're trying to portray a "bad boy, bad to the bone, and bad a$$" wanabe image.

Maybe they eat Stem Burgers.

 
They just figured out that the tattoos don't come off?

It's funny. I used to read all the guitar mags back in the day, and in every single early picture of Metallica they all had these ridiculous scowls on their faces. It was like they must have had a band meeting and decided, "Our image is we scowl all the time!". What cracked me up is after The Black Album and Enter Sandman hit and they had really made it suddenly you started seeing pictures of them all smiling. 
 
But there's an element to that phony showmanship in metal for some reason. I remember back in the early 80's I made the observation that the same people who were really seriously into metal in those days were the same ones who were really into professional wrestling. I don't think that's a coincidence.
 
Personally, I like much of the music but could do without some of the phonier theatrics. 
 
One of the things I always really liked about the Who is when after Townshend more or less gave up smashing guitars up in the mid seventies (despite fans always clamoring for it), he said something like "I just don't really feel like smashing a guitar any more". IOW, even though smashing stuff up was a shtick, it wasn't just a shtick and still had something behind it.

I'm sort of sitting on the fence about that. Yes, there's something artificial and often ridiculous. And it seems the heavier the music, the more likely to be ridicule.
 
At the same time, it's a band picture. "Acting natural" is just as phony and ridiculous. Smiling for the picture is also phony. Pretending that you aren't aware that a photographer is taking a picture that has the band for subject, a picture that is likely to be associated w/ the band and even define its image? If you've ever seen a real photo shoot, you know that it's pretty much all phony.
 
Though pretty much every band also have "candid" pictures - which I guess the fans are more likely to see, because they're usually exposed to much more than the publicity shots.
 
FWIW, Metallica did too. Quite a lot actually. If you were a fan back then, I'm sure you couldn't miss the candid shots - which usually involved lots of beer.
 
But even on albums. Look at Garage Days. Pretty silly. And the pics on the back had them having fun. There was this formal/informal thing I guess, which I thought Metallica handled pretty well.
 

 
Cliff Em All, the videotape, also featured them in a much more relaxed and natural setting.
 
As a fan when I was a teen, I used to appreciate the distinction. Gave us the impression that they were focused and had such commitment - something that you want to believe in when you're a kid and need some kind of absolutes. They looked the part. Then, offstage, you knew they were just a bunch regular dudes.
 
But it isn't just metal, is it... The difference w/ pop stars is that we are more constantly exposed to mainstream stars, they have photographers following them everywhere, so we get to see them in every possible daylight.
 
I think that's what happens w/ Metallica, btw. More and more pics being taken, so more and more "candid" ones. In the beginning, no one would have tracked down Hetfield on a shopping spree w/ his wife, so there wouldn't be pics of him wearing shorts and crocs. If they wanted a picture taken, they probably had to pay for it.

 
I'm sorry, but this just says to me, "I don't want to be taken seriously".

I remember reading an article about a Metallica concert back in the days which echoed my own impressions saying that they managed to appear both relaxed and formal - not in those words, but, anyway...
 
As I said, fans had been exposed to that side of Metallica for ever. Once mainstream medias and other publications started covering them constantly, obviously that facet was exposed quite a bit more.
 
If you get one Guitar magazine cover every two years, you probably will take it very seriously. When photographers take pictures of your every move, it's a different story.

StariseWhy are Metal Band Pics So Unhappy??

 
Eminem gets a better seat at the Sushi Bar?
 

Rain
As I said, fans had been exposed to that side of Metallica for ever. Once mainstream medias and other publications started covering them constantly, obviously that facet was exposed quite a bit more.
 
If you get one Guitar magazine cover every two years, you probably will take it very seriously. When photographers take pictures of your every move, it's a different story.

My point is those mag covers were very much posed publicity shots. Apparently all the goofy faces and scowls were the image they decided they wanted to present to the world.
 
Now in Metallica's case I think their music very much transcended that kind of thing. But I also remember reading an interview way back when with the guitar player from one of those hair metal bands (Poison maybe?). He was whining that no one took him seriously as a musician and I remember thinking, "well gee, maybe if you didn't wear women's makeup and constantly make goofy faces at the camera someone might take you more seriously".
 
For whatever reason metal developed a image thing that often seems to require bands put themselves somewhere between pretending to be deathly dreary, possessed by demons, Spinal Tap and outright camp. It's hard to pull that off with looking silly. Most of the shtick jumped the shark a long time ago because it's already been done.