Well, Final Fantasy shows how good images can be even with limited palette, so maybe we should not be so hung up on the "silhouette" part.
So, currently ideas are Sith Lords favorite stance, The Cliched Epic Sword Stance! / YEAH! and "Stand behind me" then.
Grahf's idea is interesting, but I don't thnik fitting too many characters in one image would pan out well for a logo. Cover picture, maybe. But logo, I think should show Kyon, being Big Damn Hero.
Still, notion that Kanae is one of the most important factors in the story and Haruhi is, well, Haruhi, is good one.