Well to produce a hash images are scaled down to a picture with a Pixel amount from the Power of 2 (64 pixels being the smallest with good results). Before and After resizing certain Operations are applied to get better results. Depending on the Operations used the accuracy and time to compute changes. Sample operations are, convert to grayscale, Discrete wavelet transform, Discrete cosine transform, etc. There is a Python libary that i am using : https://github.com/JohannesBuchner/imagehash
On that github repro are also links to webpages on how they work and how effective they are.
Due to these algorithims slight changes like jpeg compression artifacts, rescaling, slight cropping do not affect the hash that much. Cropping does still affect the hash the most ill try out if your Idea or similar techniques work.