People tend to forget that a lot is added in post-production. Case in point, a deleted scene from the first Avengers movie that was never completed:
Beyond the fact that there is no background in parts, his gear (in particular the scepter) looks extremely fake and plastic-y. It doesn't have the level of shine and detail that we see in the final product.
Bottom line. We won't really know what Shazam looks like until we get a teaser/trailer.