From Sega Retro
Sega Retro aims to use screenshots of the best possible quality when depicting games and videos. For most video games this is typically done by running the game in an emulator and using the emulator's own screenshot feature. If the emulator has no such capability, you can manually capture the screen by pressing Print Screen and pasting it into an image editing program. THIS SHOULD ONLY BE DONE AS A LAST RESORT! The screenshot should be saved as a PNG and optimized using pngpress (Windows) or PNGPress (Mac OS X). Remember to disable any filters when taking a screenshot (except for contrast/brightness improvement) and save the image at the correct resolution.
Sega Mega Drive
Resolution should be 320x224, or less commonly 256x224.
Sega Master System and Game Gear
Resolution should be 160x144 for Game Gear. Master System is 256x192 by default, but the leftmost 8 pixels are nearly always blank, so they can be cropped for a 248x192 resolution.
Nintendo Game Boy Advance
Resolution should be 256x192 per screen (256x384 total).
Neo Geo Pocket Color
Resolution should be 160x152.