The painter has the advantage that he's a human drawing an image that will be perceived photorealistic by other humans, so his own "is this photorealistic yet"

The painter has the advantage that he's a human drawing an image that will be perceived photorealistic by other humans, so his own "is this photorealistic yet" function is the same as mine and yours, i.e. he can alter the image until it looks photorealistic, even if it is not objectively photorealistic.