Wii

Emulators
Use Dolphin, which also supports GameCube games.

Screenshots
The Wii 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 Wii, though.

You may use JPG or PNG for Wii screenshots.

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