Computer expert Neal Krawetz has conducted extensive image analysis on many of the propaganda videos released by al Qaeda's media arm as Sahab. In many cases, he has found that the tapes were likely doctored to give a false impression of the location of the speaker, whether it be al Qaeda's No. 2 Ayman al Zawahri or American al Qaeda Adam Gadahn.
Krawetz found six different layers of video elements in this JPEG of Zawahri from July 2006, indicating some elements of the video are older while others are newer. (Hacker Factor)