Doesn’t matter. As a simplistic example, if you send the image through SHA-256 it doesn’t matter how many bits are predictable — as long as there are at least 2

Doesn’t matter. As a simplistic example, if you send the image through SHA-256 it doesn’t matter how many bits are predictable — as long as there are at least 256 bits of entropy in the image, the output will have those 256 bits of entropy smeared across the output evenly (barring breaks in the hash function itself).