may you please elucidate your scheme ? in my naïveté, it seems to me, that hashing etc would not help at all.
I'm familiar with location-sensitive hashing[1], which acts like a dimensionality reducer but, instead of receiving as input the image itself receives a feature vector extracted from it (or something analogous that allows for similarity assessment).
[1] https://en.wikipedia.org/wiki/Locality-sensitive_hashing