Calendars generally use horizontal pictures.
Most people shoot the majority of their pictures horizontally because that is the way the camera is.
I'm apparently a freak. I would say that I do about 50% of my pictures vertically.
Since I can't just pick only 12 pictures that I like anyway, I think I'm going to have to design each calendar page using 2 or 3 pictures... Then I can use some of my verticals since many of them are my favorites.
Anyhow, if you want to see what some of the pictures I'm considering (and put your two cents worth in on which ones you like) I've got a temporary gallery of them all grouped together here: http://www.dreamflyte.com/gallery/temp
*edit* mcbarnes1970 gave me a good idea. Drop a comment right into the gallery for the ones you like. :) And of course, mention here that you did so I know to go look, and because I'm a comment attention whore. *grin*