A less-celebrated painter I've been having fun exploring lately is John Martin (1789-1854), whose paintings seem like production art for unmade epic films. They're a bit kitschy, but the scale on display is tremendous. I can imagine Roland Emmerich eying these paintings with envy.
Belshazzar's Feast (which provided inspiration for INTOLERANCE):
Pandemonium (from PARADISE LOST):
The Great Day of His Wrath: