Spectrum Computing

Third figure from Amaurote and Rebel Planet is from Vallejo too ofcourse:)

Boris Vallejo, A Private Cosmos

Did some similar research couple years ago about authors of original art, which was used as reference for loading screens (and not only):
https://zxart.ee/eng/graphics/tags/alfonso-azpiri/
https://zxart.ee/eng/graphics/tags/louis-royo/

they are also authors of cover art basically.