And another form of balance would be in the dimensions of the image. Generally a vertical picture does better to have strong vertical elements in it (trees, buildings, people standing, etc), with some "balance" created by the addition of horizontal elements, and vice-versa for horizontal pictures. The best thing to do is to decide on a theme, like will it be predominantly vertical, or horizontal. Your theme, be it horizontal, vertical, or diagonal, then, should be the dominating element in he picture. This is largely opinion on my part, but most of it has come from alot of what I've read about art in general. There are always exceptions, and you don't want to become a slave to following the art "rules", because in the end, if it LOOKS good, it IS good.