The way you look at an object can affect how you see it. Sometimes there are two images in the picture, but you can only see one at a time so your brain chooses one. At times this is due to the background. For example, do you see the example below as black lines on white paper or white lines on black paper?
At times, your brain has trouble determining which object is in the background.
Once in awhile your brain has to supply missing information. For example, if you place spots on a piece of paper in the right place, your brain will see it as a dog.
Another thing that your mind does is deal with too much information. This can best be seen in ambiguous images where you can see two different images in the same picture, depending on how you look at it. To learn more about this type of optical illusion, click here.