www.localarcade.com/screenshotarchive.
Snaps, boxart, cart pics downloadable as complete archives per system rather than one at a time.
Systems include:
NES, SNES, GB, GBC, GBA, N64, Virtual Boy, Sega MS, Sega Genesis, Sega32X, GameGear, Atari Lynx, Atari Jaguar, TurboGrafx/PC Engine, Atari 2600, 5200, 7800, NEoGeoPocketColor, Intellivision, ColecoVision, ZX Spectrum and Visual Pinball.
Essential site if you're setting up a front-end.