The person who is allegedly Assange is referring to a version of this public domain photo, which had been cropped into a head shot by other Creative Commons contributors:
Color me disappointed. Apparently, he believed this poorly lit wind-swept outdoor pic was a depiction more "suitable to [his] public role."
If this request actually came from Assange, it simply underlines just how much his efforts are driven by a huge ego and thirst for fame, rather than anything remotely resembling altruistic intent. If it didn't come from him, one wonders how it's managed to remain up on Wikipedia, attributed as his direct contribution, for more than six years now.