A year or so again I had an idea for a search program for pictures on your PC. You would select a picture of someone of whom you had multiple pictures scattered across your disk and the program would find all pictures of that person. I have not followed up on the idea, but there is no doubt that it would be useful to search for pictures by their contents and not by their filename; especially if you have folders full of image files that have the camera assigned name.
Google Image Search has taken the first step towards implementing this idea. You can specify that you only wish to see pictures that have faces. Compare these two search results for “Heroes”; all and face only.
It still conducts the search based on the filename, but looks at the actual image to select those with faces. It still cannot identify the person in the picture, unless the filename includes that information.
This new functionality is not yet supported by the Google UI. You need to conduct a regular search and then append the following to the URL: &imgtype=face.