I'm thinking of making a website that explains about games (walkthroughs, level-passing guide, etc.) I would like to include screenshots of the game content. Will screenshots violate some kinds of copyright laws?
Screenshots of games will include copyrighted game assets in them. All screenshots of a game will be derivative works of the game, which gives the game creator copyrights over the screenshot.
Screenshots of games usually also involve some creativity on the part of the player. When you arrange the screen a certain way within the game to compose the screenshot you also get copyright.
Even with lots of copyrights on them, screenshots may be able to be used on websites because of "fair use." That is, they are used in a transformative way (ie not to make another game), and provide commentary or review of the game. See Legals 101: The Legality of Video Game Screenshots – Reaper Interactive