[syrrim]

This is in general an unsolvable problem, and the only possible security is through obscurity. Images are on a spectrum, and so in order to hash an image, you must quantize the spectrum. Between two images, there is a continuum of mixed images. Using bisection, you can identify the point where it goes from matching to not matching. You can then upload the non-matching variant. Because of the way hashing works, this will be indistinguishable from any other non-matching image. Therefore, this creates an easy method to bypass facebook's filter while sacrificing minimal quality. As long as facebook matches against hashes, there is way to prevent bypassing of this form. All they can do is make it as difficult as possible.