Sometimes I think the reason there are so few foreign, and by foreign I mean non-american, movies in the database is that people confuse a good snapshot with a relatively easy one. I mean, it seem that, if they can't guess it right, then it's the snapshot's fault, so it doesn't deserve a good rating.
My personal rule is to try to see if the image brings something that is characteristic, unique of a movie, even if I don't know it. So a snapshot of an actor over a normal background doesn't deserve a good rating, because it can belong to any of several movies. On the other hand, an image of an alien, a well made-up character or a specific background, can only be of one specific movie, so I rate it well.
Besides this rule, there's also the beauty of the shot, that to me adds up some pointa as well.
I'm a doing it wrong? Because I sometimes feel that snapshots of little known movies do not go to the Feature Filme database due to a lack of knowledge by the users.