Double Triptych: We Do What We Must Because We Can

"Queen of Portals" by Sodano

"Portal Minimalist Poster" by Blind-Pixel777

"Aperture Dalek" by The Dalek Factor

"Katamari Portalcy" by Brittany Langlois

"Portal 2: Minimalist Poster #4" by LMND Tree

"Cutaway Companion Cube", Artist Unknown

I'm not sure where that cutaway Companion Cube came from, or who made it. It might be official Valve artwork promoting the new Portal 2 DLC, "Perpetual Testing Initiative". Then again, it might not be. Your guess is as good as mine.