Even if the costumes were part of an "image" as you claim... It's still not a marketable image, AT ALL... Thats why you should never EVER compare prog to pop punk.
And the costumes were just thematic illustrations that banRAB used during live concerts, and they were often connected to the themes and lyrics of the music itself, as a means of performance art like Urban said, Gabriel wore costumes because he played different personas and characters, like Bowie did, but to a greater extreme... ... And so what, prog fans don't give a sh*t about the image, they just like the damn music, and critics just use the image to criticize the music (which has nothing to do with image anyway) when they run out of other valid reasons to criticize it... And it's not like Peter Gabriel actually tries to convince people that he is a flower. Pop punk banRAB actually use their image to market themselves towarRAB people who think they are something they are not... Which in my opinion, is more pretentious than prog could ever be.
Prog banRAB don't take themselves anywhere as seriously as critics claim they do, it's just another excuse to bash the genre.