How to identify the image visually?

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Hi, I take jpeg image from url and cut it on parts and save it with name of md5 value from bytes stream. But this the approach only works in perfect conditions, because as practice shows, the resulting image does not differ visually but have different file sizes and md5 hashes too. I even compared using the (subtract) and it showed no differences. How to use opencv in to detect similarities or something like that? What would the next time to understand that this picture is already there.