If this reads like greek, let me dumb it down. You get the hash of the final result from the trusted server and hash is checked. Because of this you will never

If this reads like greek, let me dumb it down. You get the hash of the final result from the trusted server and hash is checked. Because of this you will never get an invalid image. There are also some clever tricks to make sure no one can force you to start over from scratch by sending wrong data. But that's more of a detail.