Just randomly stumbled on this, but:

https://www.gog.com/en/game/disciples_2_gold

Does anybody know what the deal with the 2nd screenshot is? First, the unit portraits look as if they are from Disciples 1? But not quite. Next, the portraits are absolutely not the units shown on the battlefield. No idea why a Thug, Wolf, Centaur Lancer and a Forest Elf are have portraits of 2 angels, a wolf-thing and the thug has a portrait from Disciples 1. Lastly, the unit stats at the bottom show the type of attack a unit uses (Warhammer, Daggers etc.), which is not visible in the game without checking out the unit details.

The first screenshot also seems off, where the layout of the briefing screens is different than in the actual game. Could those be some early development screenshots that were kept in the official promo materials to this day?

Its generic elf not angel. There is on store some pre-pre-release and too early. There is more screens from all stages of earliest D2. In some placeholder images almost unique for differ units (one per sub-faction or type), in some else just 1 oldest placeholder pic.

Disciples 2 originally look more like D1.

There is many saved screens from it but there was far more and need deep digging with Internet Archive and Internet Web Archive for web and magazine previews.
GameSpot preview is one of main sources for such early screens. But there was much more sites some have exclusive screens from devs. Start from Web Archive of Disciples 2 official website news page and linked reviews there.

here big list of earliest pics/hidden content for all of Disciples incl Liberation.
[url=http://disciples.fandom.com/ru/wiki/%D0%A1%D0%BF%D0%B8%D1%81%D0%BE%D0%BA_%D0%BA%D0%B0%D1%80%D1%82_Disciples_II:_Blades_of_War]http://disciples.fandom.com/ru/wiki/%D0%A1%D0%BF%D0%B8%D1%81%D0%BE%D0%BA_%D0%BA%D0%B0%D1%80%D1%82_Disciples_II:_Blades_of_War[/url]

Blades of War use earliest version (pre-almost-release)
[url=http://disciples.fandom.com/ru/wiki/%D0%A1%D0%BF%D0%B8%D1%81%D0%BE%D0%BA_%D0%BA%D0%B0%D1%80%D1%82_Disciples_II:_Blades_of_War]http://disciples.fandom.com/ru/wiki/%D0%A1%D0%BF%D0%B8%D1%81%D0%BE%D0%BA_%D0%BA%D0%B0%D1%80%D1%82_Disciples_II:_Blades_of_War[/url]
Disciples official site use same releasecandidate game version arts.

Plus Disciples 1 Gold Edition and Disciples 2 Gold edition have more. But not on GOG. Steam version of D1 have two bink-exe from CD Gold which is promo for upcoming Disciples 2 back in 2001.
D2 Gold content partially included with GOG.
Here is more from discs
[url=http://disk.yandex.com/d/CuNT609GyvqbxQ/Series_Disciples_%5BGOG%5D/disciples_series/disciples_2_gold/misc_content/_old/_Preview%20(non-GOG%20content!!!)(old%20CD%20Gold]http://disk.yandex.com/d/CuNT609GyvqbxQ/Series_Disciples_%5BGOG%5D/disciples_series/disciples_2_gold/misc_content/_old/_Preview%20(non-GOG%20content!!!)(old%20CD%20Gold[/url])

Thanks for the links. I suspected those were some pre-release, dev screenshots, but interesting they are still present in the official promo materials on stores to this day.

The board game seems to be using the same art that was in the final game, besides the items.

As for the Gold versions, I actually do have them on disc, which contains both (D1 Gold and D2 Gold). Specifically this:
https://im9.cz/iR/importprodukt-orig/9be/9bee40ed8a0480ac88b4cb8f94e2ee70--mm2000x2000.jpg

Not sure it has all the relevant content though, but probably does.

Its should be most complete Gold edition including content still missing or inproperly incorporated on GOG with their updates. There was 2005 Gold and 2006 Gold and GOG miss from both.

As for artworks and multimedia promo materials this edition probably have less (probable more?) than original Gold editions. Look to dsic content for Bonus and Artwork folders.

Blades of War use some release candidate models. Archmage Empire hero have a bit alternate textures in finale version. It have also some time before release alternate portret. Portret can be seen on official website (via Internet Archive). I mean it was finale style not like very old D1 style and yet it was changed few times aleven after whole game design changed.
Potions probably too.

In general yes. Card game almost identical to finale version.

Hmmm. I just looked on the Gold disc I have, and it seems it has none of the extra content. No Bonus or Artwork folders at all, even after I installed the game. Also, the Gold edition I have is from 2004.