Cool find! I think this is probably the answer to who she is. I also think the fact that Link's back is facing, and the girl is standing where the Master Sword would be placed on his back in a sheath has something to do with hinting at who (what?) she is.
It was never announced to be the game's cover art, it was presented as concept art. It could just be a piece someone did to get a feel for the new character, or to get a colour palette down. It does have that TP promo art feel to it though.
Very impressive, but I have to say I don't know why everyone says Link is considerably older in the text descriptions from the conference. He looks pretty much the same as TP Link to me. Maybe it was stated by Miyamoto that he was considerably older, and reporters just incorporated that line...