Similarity: This poster done by Kevin Finn is a great example of the principle of similarity. It is evident that all the red rectangles on the page are the same colour, size and shape, and are therefore interacting with each other as the first few rectangles have been knocked over. This suggests that as time goes on, all of the dominoes (or however you would like to perceive them) will eventually fall over.
Figure/Ground: This poster for the film Sherlock Holmes was done by Olly Moss and perfectly demonstrates the optical illusion that can occur in a figure/ground piece. If you look closely, you will be able to flip between the white outline of smoke coming out of the pipe (which could also be an example of continuation) and the red figure on the left of a man in a hat.