Professional Screenshots
From YoYoGames Wiki
If you do a search on YoyoGames, you are certain to find Games with screenshots that make you interested in the game, and ones that turn you away from it.
Why Professional Screenshots matter
Why do you need a good-looking screenshot when it has no real influence on your game? Because the screenshot is the first thing that anyone will notice when they see the game (unless their computer has problems and takes ages to load the pictures). See the pictures below - which makes you want to play the game.

This logo for 2Dcube's Karoshi 2.0 is well made and professional looking.

This "Pacman Ghost" is harsh on the eye, looks scaled up, and is much too common an image on YoyoGames. Plus doesn't say ANYTHING about your game (other than that it's probably not something worth playing), is over-used AND is a simple sprite that comes with Game Maker.
See how it makes a difference?
Tips for making your screenshots decent
- DON'T use a sprite or a spritesheet
- Make sure it's a) a screenshot of your game or b) a logo or advertisement picture made for your game
- Make sure you have permission from its creator (if you didn't make it) to use it
- Make it related to your game
Check the WIKI entry about Creating Screenshots [1] if you don't know how to make them.

