Last time I wrote about the different things that make up the elements of an image - things like shadows and reflections, shapes left by the space between one object and another, or between one object and the edge or corner of an image (unless you print in circles - it's been done).
The shape of an ojbect is probably its most obvious characteristic. What may be less obvious is that the shape of the shape affects the quality of the image. The most stable shapes are rectangles aligned with the edges of the image, horizontal ones being more stable than vertical (they can't fall over).Objects that are just a little off rectangular (trapezoidal or parallelograms) can be ever so much more interesting. The other night I was looking at a Matisse print and it consisted of a series of rectangles within each other and it was the slight "misalignment" of one rectangle on another that made things interesting. The lines weren't perfectly straight either and the rectangles really had a sense of life. Parallelograms suggest action while trapezoids suggest perspective - ie. one part is closer than another part of the shape.
It may be true that circles roll better than ovals, but they sure look a lot more stable.
Imperfect circles breathe life into an image and suggest change over time. At least one edge of a triangle is going to be a diagonal line which has energy and movement.
Triangles with a wide base and pointed top also suggest a receding perspective.
You may have different meanings for the usual variety of shapes, based on your experiences and that's ok, just so long as you take the shapes into consideration. Remember too that a change in camera position can radically affect the shapes in an image - narrowing them or rounding them, making them lean or not.
Next time - relationships between the shapes.