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Sorry, I am not to answer what you are explicity asking for, but I want you to know about CBIR, because I think this is the right thing you are trying to implement.

"to retrieve the best matched image from the image database given an input image"

You should clearly define what is the best matched image, because identifying best match could be based on: low-level features (color, image descriptors, etc)
or high-level features (represents semantically meaningful concepts in the image). This is called semantic gap (http://en.wikipedia.org/wiki/Semantic_gap)

I recommend you to read about CBIR (Content-based Image Retrieval). Here for example, is the list of CBIR engines. This list can give you a quick start: http://en.wikipedia.org/wiki/List_of_CBIR_engines