Picture Plane Space
A picture plane is the flat 2D plane on which your image lies. Corners, edges and centers are not natural to anyone but painters, photographers and cinematographers. Don't believe me? Well show me the where the four corners of your vision are! Even centers are, perceptually, very fugitive things.
There are some very broad dos and donts in picture plane space. They all revolve around one necessity: that the impression of flatness be avoided.
Avoid corners: A corner draws attention to the rectangle of the painting surface. Unfortunately, they have a magnetic effect upon the lines we draw, and we often find that our lines have been pulled into the corners without consciously intending them to be.
Avoid centre points: Again, the centre point of a rectangle is a feature of the flatness of a painting, not its depth form. On the whole the point centre should be avoided.
Avoid centre lines (usually): Here the rules are not so strong. Middle verticals are frequently used in painting, especially portrait painting. Middle horizontals are less frequently seen, and are generally best avoided, especially in landscapes. The natural place for a horizon in towards the lower regions of the painting, less frequently its upper regions and rarely its exact middle.
Avoid symmetry: Clearly symmetry is a very flat thing. Also avoid repetition, which can be regarded as a form of symmetry.
Avoid touching: Avoid what? Touching? Yes. By this I mean avoid delineating regions in such a way as their perimeters touch. Such regions are spatially confusing, are they behind or in front of each other? They should instead overlap or be separate. The middle rectangle pair in the image below is in a certain relationship: one square is clearly behind the other. The example on the right in an uncertain relationship: which is behind the other? It is confusing. The example on the left is also in an uncertain relationship, but is not confusing.