MC doesn't have a human form.
She was the pet spider in the class room. She became a spider in the new world with a err "proper soul" since D gave part of hers to make it possible. When she finally evolves in an Arachne she has silver hair...when she evolves into a god she has the same silver hair
So like Jjsnage said, the girl on the cover is D. It's just the MC has D's memories and *thinks* she looked like that. In reality she has never looked like that.
the "form" is exactly similar apart from the eyes and silver hair when she ascends, again i was just mentioning about her form, never said it was not D on the cover
if anyone wants to be ahead of the manga its chapter 165 in the light novel I think but it's best to start at chapter 135 where she re-encounter Alibaba the earth dragon