Spoilers
I kind of hate to spoil the random image thing I've got on this site by just showing them all off at once, but I obviously have to credit the artists where I can. There are a few background images that aren't included here—in cases where I can't find a meaningful credit, or where I actually made the image myself—so I guess there might still be a few surprises!
Also, I need to thank Aeron Alfrey, whose excellent Monster Brains blog provided loads of these images. No better place on the Internet to get awesome high-res scans of cool monster art!
Backgrounds
Cenni di Francesco — The Temptation of Saint Anthony
Cover art from Popular Detective (April, 1937)
Ed Valigursky — cover art from The Best from Fantasy and Science Fiction: Sixth Series
Frank Frazetta — cover art from Lost on Venus
Frank Frazetta — cover art from Swords against Darkness
Greg and Tim Hildebrandt — cover art from Earth's Last Citadel
Hex map from Castle Book II
Hex map from Gamma World (3e) boxed set
Joos van Craesbeeck — The Temptation of Saint Anthony
Ken Kelly — color insert from Marchers of Valhalla
Ken Kelly — color insert from Red Nails
Ken Kelly — color insert from The Hour of the Dragon
Ken Kelly — color insert from The People of the Black Circle
Ken Kelly — Hornetroid toy package art
Ken Kelly — Terraphant toy package art
Malcolm Smith — Cover art from Imagination Science Fiction (June, 1956)
Martin Schongauer — The Temptation of Saint Anthony
Max Ernst — The Eye of Silence
Peter Paul Rubens — The Fall of the Damned
Thomas Cole — Manhood, from The Voyage of Life series
Tim Hildebrandt — poster art from The Deadly Spawn
Tim Hildebrandt — "The Demon's Swamp", from the Realms of Wonder calendar
Tim Hildebrandt — "The Firepit", from the Realms of Wonder calendar