Earlier today halelift2.net editor posted on a forum about an image he was emailed from Gabe himself:
Most curious.
However, it wasn't long before other reports surfaced. The joystick editors
tweeted about a similar email, with a different image.
Also of note, over the past day Aperture Science login screens have started to crop up in other games! Most of them all from the Potato sack indy pack on steam:
Save 75% on The Potato Sack on Steam
These login screen pop open in the steam browser overlay after you've completed a certain hidden task. So far people have found them in Audiosurf, Cogs, Killing Floor, BIT.TRIP BEAT, Toki Tori and Super Meat Boy, details of each can be found here:
Portal 2 - Valve ARG
Yet more website editors have also received emails from Gabe with similar images only hours after Joystick and HL2.net editor reported their cryptic gifts.
This one's from MacRumours:

"sorbet coopers.jpg"
then MacWorld.com editor David Chartier tweeted about his email here:
Twitter / David Chartier: Looks like Valve is beginn ...
It appears all of the image names are anagrams of "REBOOT PROCESS" and some people have started to jig-saw them together, here's the current "solution" once most of the image colours had been inverted:
Clearly more images are yet to surface, but no one knows which websites they've been sent to. The sequence of numbers or perhaps some other final image analysis is sure to lead us down yet another rabbit hole. I think this is only the tip of the iceburg.