The most common type, should show all the gameplay variations, special sections like end-of-level bosses included.
This will show the title art, and occasionally the game credits.
Character / portrait
In games with selectable character, this may be needed to show the available roster (in case character selection screen screenshot is not sufficient). This is the only type that is accepted to exceed the "five images" rule.
Only the character images, extracted from the background. Not used much.
The package containing the game media. Useful to view copyrights and sometimes the release date, just like the title screen.
This could be in-game graphics, covers, prints, or other artistic media.
Video / animation frame
For intros, outros, and in-game animations.
Cabinet / Card / Hardware
The hardware part of the game, depending on the platform.
Not used so far.