The lockers in the hallway exemplify the grouping principle, similarity. The lockers are exactly the same, which makes our brain conclude that they belong in the same group based on their similarities.
This edited photo of Olaf against a black background is an example of figure-ground. In this photograph, Olaf (the figure) stands out from its surroundings (background).
Proximity is the grouping principle shown in this image of the bookshelf. Because the books are close to each other, it demonstrates that they are in the same group.
This image is an example of closure because our brain is seeing a triangle when it's really three halves of circles placed together. Closure is our brains tendency to look for the whole rather than only the parts.
Continuity is what is being demonstrated in this image. I chose the staircase because the definition of continuity is, "Once an object appears to be moving in a particular direction, your brain assumes that the movement continues unchanged." When I am on the staircase and I'm looking down at my feet, I often keep on moving my feet as if there is another step, at the end of the staircase, when in fact there is not. Most likely then not, I end up tripping. In this image it looks as if the staircase keeps on going up because of the way that it is cut off.