It's not a simple filehash.
We've had TinEye - capable of taking an input image and finding cropped variations across the net - for years and years. It's developed in the meantime, but this is far from new technology.
The hashing is based (AIUI) on the variation between pixels in "key" locations (of which there are more than a few), so cropping the image won't help. Even contrast tweaks have to be fairly extreme to throw it off.
All that said, it's far from perfect.