A hash would change if even one bit changed in that file. This could be from corruption, automated resizing by any photo processing tools (i.e., most sites will resize photos if you give them one too big), saving a lossy file time again (adding more jpg), etc… This is why there aren’t many automated tools for this detection. Sites that have tried by using skin tones in a photo have failed spectacularly.
I’ve never heard of this FBI middleware. Does anyone have the link to this? I’d like to understand what tools are available to combat this as I’ve been considering starting my own instance for some time now.