Why do you think Bernard looked anything like Arnold? After all, It's only the mind that matters.
Three things made me think that:
(1) Bernard couldn't see Arnold in Dr. Ford's old-timey photo (first time we see it). Hosts are programmed for "it looks like nothing", if that thing would reveal their nature. If Arnold looked different than Bernard, it would be just fine for Bernard to see Arnold because it wouldn't reveal Bernard's true nature in any way--it would just be a picture of some people. It would also be fine for us, the viewing audience, to see Arnold because if they looked different it wouldn't prematurely reveal the Arnold/Bernard plot twist.
(2) When we see Dr. Ford's old-timey photo with Arnold in it (second time), the blank space to the right has Arnold appear, and it looks like Arnold/Bernard.
(3) There is a scene where Dolores sees Bernard for the first time and excitedly calls him Arnold.
Granted, #3 can be easily explained away because she seems to be seeing Arnold anyway, even when he isn't there at all. But still, it was one of things that makes me think that Arnold/Bernard look the same.