The art was created in March of 2008. One moderator wondered if Valve would "officially comment on these pictures?" The album containing the works has since been pulled from the net.
Fansite Lambda Generation grabbed the art from Andrea Wicklund's Picasa portfolio before they got taken down. They've uploaded watermark free versions for fans.
Officially the concepts were of Half-Life 2: Episode Three but that was essentially canned in favour of full-fledged development of Half-Life 3. "I know who the source is, but I'm not going to give it away just yet for exclusivity reasons," Valve moderator Smash.
Fellow Steam officer of forum law, Oscar Sierra chipped in. "Well, tis legit," he said, "Awesomes. Hats off to ValveTime.net for the scoop. I wonder will Valve officially comment on these pictures?"
"These are real," he added later. "Now keep in mind it is only concept art - but what has been posted are not fake."
Check outfor the artworks.