Box art has been revealed

[Hikamaru]

Oooh...but they might not be Solgaleo or Lunala. Necrozma supposedly has the ability to bend light to its own will, so it could possibly in fact be Necrozma creating an optical illusion so that it resembles Solgaleo and Lunala, or even it could actually be a different Pokemon entirely (unlikely). I'm personally more hinged towards Necrozma-Sun and Necrozma-Moon as the names of the new forms, rather than Ultra Solgaleo and Ultra Lunala.

Okay, that may actually be an interesting theory you have there. Notice how these Solgaleo and Lunala are like very lit up, similar to the so-called "Radiant Sun" and "Full Moon" phase forms that happen when they use Sunsteel Strike and Moongeist Beam.

I'm however betting that this theory may not be the case and that it's just Solgaleo and Lunala with armour formed from Necrozma's body.