I saved the picture and expanded it using Corel Photopaint. Not much luck due to poor resolution. But it appears to me that the discolored area near the picture's center is NOT an exhaust trail but a cloud of dust resulting from the aircraft's initial impact in the vicinity of the helipad. Also the debris that is well away from the building indicates that the aircraft was shedding parts prior to impacting the building. That would support the official account of events. Also no modern jet leaves an exhaust trail that is as dirty as this one appears to be. The last commercial passenger jet that had engines which polluted that much was the 707 and they haven't been in service for over 20 years.
Final item - I went to the web site which claims to prove that no 757 hit the Pentagon
http://www.asile.org/citoyens/numero13/pentagone/erreurs_en.htm
Wonder of wonders - look at the photograph for question #5. Damage to the building coincided EXACTLY with the overlay of a 757. Yes, the damage sustained by the building in the area struck by the plane's wingtips is substantially less than the damage near the center of impact. That's because the structural strength of a wing at its tip is far less than at the wing root. Second, the total mass of the wing is concentrated near the fuselage and not at the outboard ends.
People who point to these photos as proof of their silly conspiracy theories are either ignorant of aircraft construction, or lack any understanding of fundamental physics. Then again they could be so determined to find a conspiracy that their ambition blinds them to the facts. Or they could be liberals who, having given up on presenting kerry as a war hero, see an opportunity to dupe a few lunatics into voting Democratic this November.