GameCube

Emulators
Use Dolphin, which also supports Wii games.

Screenshots
The GameCube isn't emulated perfectly. If you believe (or know) that the game you're interested in has graphical glitches in Dolphin, don't submit screenshots for it from Dolphin. You can use a video capture device and a real GameCube, though.

You may use JPG or PNG for GameCube screenshots.

Accepted resolutions are 640x480, 640x448, 512x448, and possibly others.

In Dolphin, set the emulator to use the game's native resolution by going to Graphics Configuration | Enhancements and setting Internal Resolution to 1x Native (640x528).