What is perceived when two images are combined?

When two images are combined three perceptual outcomes are possible. One, they can be perceived as they are--separate and independent images. Two, they can become perceptually fused into a new image. Three, one image may dominate, or mask, the other. These possibilities are demonstrated with a variety of images and it is proposed that it is their spatial correlation rather than their spatial frequency similarities or differences which is critical in determining the particular outcome.