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Google "reverse image search" for some interesting results, including:
Inside Google Search · Search by Image
TinEye Reverse Image Search
Top 10 reverse image search engines | Comptalks

Again, what is similar? If you input a picture of a red car, which picture is similar- a picture of a red ball, a picture of a blue car, or a picture of the same car from a different angle? How would...

Can you define "perceptual similarity"? How would a person determine how similar two images were?

What you're talking about is not trivial- google something like "java image processing" for...