Acceptable screenshot resolutions
PCs
Amiga
320x200 / 320x400 / 640x200 / 640x400 NTSC
320x256 / 320x512 / 640x256 / 640x512 PAL
The Amiga is notorious for changing the screen size between, or inside, games. Look out so that you don't crop overscan areas, since some games exceed the default PAL and NTSC resolutions. Do not use GIF for Amiga screens, unless you are absolutely certain that less than 256 colours are used, including rainbow effects in the background. PNG is the preferred format.
Apple II
560x384 / 280x284 color png/gif
Apple IIgs
320x200 / 640x200 png/gif
Amstrad CPC
320x200 png/gif
384x270 (overscan)
640x400 (hi-res)
Atari 8-bit
336x240 png/gif
Being the predecessor of the Amiga, the Atari 8-bit computer is also prone to change the size of the screen. The above resolution should capture all the overscan area, but cropping is welcome if you get big borders at that resolution.
Atari ST
320x200 / 640x400 png/gif
BBC Micro
320x256 (low-res)
400x300 (emulators, using overscan)
640x250 (mode 7)
C128
320x200 (40 column mode)
640x200 png/gif
640x400 (interlaced)
C64
320x200 png/gif
384x271 (overscan PAL)
384x265 (overscan NTSC)
DOS
(various text resoutions, unlisted)
720x348 (monocrome Hercules)
640x200 (monocrome CGA)
320x200 (4-colour CGA)
160x100 (16-colour - Tweaked CGA)
160x102 (16-colour - Tweaked CGA)
640x200 (16-colour - Tweaked CGA)
320x200 (16-colour EGA)
640x200 (16-colour EGA)
160x200 (16-colour EGA)
640x350 (16-colour EGA)
640x400 (16-colour Extended EGA)
640x480 (16-colour Extended EGA)
720x540 (16-colour Extended EGA)
320x200 (256-colour VGA)
640x480 (256-colour PGA)
704x528 (16-colour ModeX VGA)
736x552 (16-colour ModeX VGA)
768x576 (16-colour ModeX VGA)
800x600 (16-colour ModeX VGA)
256x200 (256-colour ModeX VGA)
256x240 (256-colour ModeX VGA)
256x256 (256-colour ModeX VGA)
256x400 (256-colour ModeX VGA)
256x480 (256-colour ModeX VGA)
320x200 (256-colour ModeX VGA)
320x240 (256-colour ModeX VGA)
320x256 (256-colour ModeX VGA)
320x400 (256-colour ModeX VGA)
320x480 (256-colour ModeX VGA)
360x200 (256-colour ModeX VGA)
360x240 (256-colour ModeX VGA)
360x256 (256-colour ModeX VGA)
360x400 (256-colour ModeX VGA)
360x480 (256-colour ModeX VGA)
640x200 (256-colour ModeX VGA)
640x240 (256-colour ModeX VGA)
640x456 (256-colour ModeX VGA)
640x400 (256-colour ModeX VGA)
800x600 (16-colour SVGA)
1024x768 (256-colour SVGA)
1024x768 (256 colour XGA)
640x400 (256-colour VESA)
640x480 (256-colour VESA)
800x500 (256-colour VESA)
800x600 (256-colour VESA)
896x672 (256-colour VESA)
1024x640 (256-colour VESA)
1024x768 (256-colour VESA)
1024x768 (UGA (very rare))
Note that "Super VGA" is non-standarized and can vary greatly witin this range. VESA is considered for our purposes to be the standardized SVGA.
Dragon 32/64
256x192 png/gif
Linux
MSX
256x192 (16 colors)
512x212 (16 colors out of 512)
256x212 (256 colors)
256x212 (19268 colors)
PC Booter
TI-99/A4
256x192 png/gif
VIC-20
VIC-20 screen size varies a lot, just make sure that the aspect ratio is correct (pixels are usually twice as wide as they are tall) and that borders are not larger than necessary.
Windows
320x200 / 320x240 / 640x400
640x350 / 640x480 / 720x384
800x600 / 1024x768 / 1152x864
1280x720 / 1280x800 / 1280x1024
1360x768/ 1366x768 / 1440x900
1600x900 / 1600x1200 / 1920x1080
Windows 3x
ZX Spectrum
256x192 png/gif
CONSOLES
3DO
Atari 2600
320x200 / 320x225 / 320x240 png/gif
Atari 5200
336x240 png/gif
Atari 7800
320x200 / 320x225 png/gif
ColecoVision
256x192 png/gif
Dreamcast
640x480
GameCube
640x480, 640x448, 512x448, possibly others
Genesis/Mega Drive
320x224 / 256x224
Intellivision
320x200 png/gif
Jaguar
Neo Geo
NES
256x224 / 256x240 png/gif
N64
PlayStation
320x240 / 512x240 / 640x240 / 320x480 / 512x480 / 640x480 (NTSC)
320x256 / 512x256 / 640x256 / 320x512 / 512x512 / 640x512 / 720x576 (PAL)
PlayStation 2
640x480
PlayStation 3
NTSC
640x480 (4:3) / 720x480 (4:3) / 854x480 (16:9) / 1280x720 / 1920x1080
PAL
720x576 (4:3) / 1024x576 (16:9)
Saturn
Sega 32X
320x224 / 256x224
Sega CD
320x224 / 256x224
Sega Master System
256x192 gif/png (almost all games)
256x224 / 256x240 gif/png (used in Codemasters games)
SNES
256x224 / 512x448 - (used rarely)
TurboGrafx-16
TurboGrafx CD
Vectrex
360x480 png/gif
V.Smile
640x480
XBox
640x480 / 1280x720 / 1920x1080
XBox 360
NTSC
640x480 (4:3) / 720x480 (4:3) / 854x480 (16:9) / 1280x720 / 1920x1080
PAL
720x576 (4:3) / 1024x576 (16:9)
HANDHELDS
DS
256x384
Game.Com
Game Boy
160x144
Game Boy Advance
240x160
Game Boy Color
160x144
Super Game Boy
256 X 224
Game Gear
160x144
Lynx
Microvision
Neo Geo Pocket
160x152
Neo Geo Pocket Color
160x152
N-Gage
Nintendo DS
PSP
480x272
Virtual Boy
384 x 224
WonderSwan
WonderSwan Color
*ANY* CONSOLE OR COMPUTER WITH ANALOG (TV) OUTPUT
These are unaltered SMTPE 601 capture resolutions and any shot with these dimensions coming from captured analog output should be accepted
NTSC
704x480 / 720x480 / 720x486 png/jpg
PAL
704x576 / 720x576 png/jpg