I have investigated simulacra, the images that result from the human tendency to "recognize" pictures in random patterns, like ink blots or clouds. The tendency, called pareidolia, is a neurological/psychological phenomenon by which the brain interprets vague images as specific ones. The most famous simulacrum is the Man in the Moon.
I catalog many "miraculous" simulacra in my book Looking for a Miracle (1993, 34-40) and in my investigative article "Rorschach Icons" (Skeptical Inquirer, November/December 2004). The latter includes my on-site investigations of the Holy Tortilla with "Jesus" in New Mexico; the Clearwater, Florida, "Virgin"; the Milton, Massachusetts, "Madonna"; and others. See also my "Grilled-Cheese Madonna" (Skeptical Briefs, September 2008), which describes my actual examination of the sandwich in question.