The images that each site received correspond to a particular Valve game (Shacknews got Team Fortress 2, Eurogamer got Left 4 Dead, MacNN got Portal and MacRumors got Half-Life). Rock Paper Shotgun got the Steam-Mac teaser which is a big hint that Valve' successful PC games download service is also coming to Apple's machines. The dots at the bottom of each teaser image indicate a sixth image could be released. But what will it show . . and what web site, if any, will show it?
Update: We have our answer: MacWorld got the final image which is a recreation of Apple's 1984 Super Hero ad with Half-Life 2's Alyx Vance
Gallery: Valve Mac Announcement Teaser Images