First design tries to bring out the feeling of reliable. The rectangular base give a very solid feel. The shadow gives an illusion of a building like structure. The immovable feeling was created.
The second design tries to use the relationship of objects to bring out move reliable feeling. An object relying on the wall, which bring out the strong feeling of the object will never fall due to the support from the wall.
The third design tries to bring out the contrast of the feeling of unreliable. The circular object was bringing out as it gives the feeling of easily moving around. Slanted surface brings out the uncertainty feeling that everything might fall off the poster.
The fourth design tries to make the reliable object the main forces of the poster. It gives the feeling that it is a strong standing object.