So, the nice version of the cover of City of Heavenly Fire that went up on TV yesterday!
Yes, that is Clary and Sebastian. Yes, Clary is wearing white. She does wear white in the book at a certain point. There are interesting details up close about both their weapons, and if you look closely you can see the chain of Clary's necklace. Remember the covers of the books don't use the runes from the Codex or that we saw in the movie. They are pretty designs, but they don't mean anything.