Just a relatively small number of pixels would be sufficient. A secure hash function has the avalanche property that changing 1 bit in the input, has an indepe

Just a relatively small number of pixels would be sufficient. A secure hash function has the avalanche property that changing 1 bit in the input, has an independent 50% chance of flipping each bit in the output. So you just need to ensure that you have enough entropy for your desired security level. Anybody wanting to reconstruct what the camera is seeing would have to reconstruct it to the exact pixel values, including any noise. i.e. you would have to intercept the signal from the camera they are using, you couldn't just point another camera at the wall.