"Shape" refers to the geometric or organic forms that create an image. They are often composed by lines, though the lines might not be distinct.
In the first image, the viewer is immediately wowed and focuses on the ceiling. Specifically, the shape of the room. It is very unique and dramatic and is beautiful to look at. The image focuses on more than just the lines that compose the image, but on the volume of the room, which is really all about the shape.
The second image again focuses on shape. As a maternity photo, the photographer is emphasizing the belly bump that signifies a new life, a new little baby starting to grow and ready to be born. Without using shape, it would be hard to express these concepts. This photo particularly emphasizes the idea of shape, and the flowing forms beautifully portray this exciting time.
My image at the bottom also focuses on shape very simply. The shape is actually the subject as well. It is not a concrete image, as the shape is made out of a shadow, but the lines form a flowing shape that the viewer perceives is the outline of a person - specifically, the form of the photographer. The viewer's eye is led by the shape, and that is what makes the image interesting.