I found a nice version of the El Capitan Trueno art, but it's very lowres:
Source: El Capitán Trueno [Amstrad ESP]
Hires Cover scan:
Source: CPCRULEZ > AMSTRAD CPC > GAMESLIST > EL CAPITAN TRUENO (c) DINAMIC
Poster:
Original from Internet, not really complete, and not detailed:
Enhanced/Improved by Arikado:
Arikado1476 - SteamGridDB
Old (test)
New (final?)