Emergence, reification, multistability, and Invariance are all factors that contribute to how we see the images we see. In emergence you see the hidden image as a whole. It pops out from the rest of the picture if you focus on the image for a while. There can be descriptions of what is in the image but they can never be explained. It is just a fact that we can see these pictures with the help of our vision. Reificaion is also visual, and can be explained by illusory contours. This is the constructive and generative aspect of perception. In images with reification there are a lot of spatial information. In such images you see the picture as 3D or you might see some shape that really wasn't drawn in but you see it because of small hints and you create that shape in your head. Multistability occur in images that can be viewed as two different things. Depending on how you look at it you might see one image and another if you look at it differently. According to Gestalt Psychology "Invariance is the property of perception whereby simple geometrical objects are recognized independent of rotation, translation, and scale; as well as several other variations such as elastic deformations, different lighting, and different component features."
All of these factors can go hand in hand in a single image to help you see different things.