Doesn't the hashtag of a picture change by editing one pixel? Or by removing one line of pixels?
I'm pretty sure that posting an image with the comment "#thisischildporn" is going to raise some eyebrows no matter what how much you change the image.
The cryptographic hash of a file, on the other hand, is usually a little more sensitive.
When dealing with image recognition you really want to account for things like the image being resized, colours adjusted, or borders being added and removed. In this case you would want to do something like take the image, rescale it to a fixed size, reduce the colour palette and then create a hash of the resulting image. This allows you to spot matches even if the image has been scaled down, blurred or watermarked, or, in some rare cases, replaced with a photograph of a giant watermelon.
We're still working on that last part and Her Majesty regrets the unfortunate incident involving the torch bearing mob at the farmers' market.