I was wondering the same thing, when I looked at the poster. Does Chaney ever appears as himself, a la Karloff, in any flashback scenes? It's been so many years since I've seen this movie, that I can't recall. If he does, then it makes sense, in some fashion.
If he doesn't, did Universal just want his "pretty face" on the poster? I don't think that Chaney could have insisted on something like this, as he would have been under contract to Universal and had little to no say in such matters.