I'll go into it with a little more detail and try just to state very simple stuff
First off the this is the actual picture, which is almost always shown zoomed up real close attempting to make it bigger than it actually is, as even the most ill informed have a rough idea how big comercial aircraft engines are.
http://www.photolibrary.fema.gov/photolibrary/photodetails.do?id=4414
click on download orginal pic on the page to see it full size. See how small that is ? Around two feet in diameter.
This is the size of an actual 757 engine

Anybody see any problems with the picture of the turbo prop found at the pentagon and and the picture above ?
Now compare that to this, debri from a tomahawk

and similar engines from missiles, you can use the flowers and flowerpots in the back and fore ground for reference to size

Also this is the actual hole created by a 124 ft wide and 44 feet tall 757 in the side of the pentagon

Also notice the how low the hole is on the ground, just the ground floor, for it to be that low the plane had to be skiding on the belly of it's fuselage and it would still hit the middle of the first floor and leaving the grass lawn in front all dug up.

Not to mention the unheard of pilot skill required for a 757 to be brought in so low ! By Hani Hanjour a hopless pilot who could not control a single engined Cessena let alone a 757. Also why did'nt it fly into the first chance he had instead he flew by it, did a 270 degree, diving turn ending at the bottom of the pentagon wall. Is akin to driving a grey hound bus at 200kph doing a almost 360 turning perfect skid passing through a tunnel with inches of clearance on both sides on four flat tyres. I don't think even the stig could do that let alone some one who had just a few hours of driving lessons on a moped !
Not to mention according to the law of science it is impossible due to a phenoma called ground effect for an aircraft with that wingspan and the speed it was said to be comming in to fly that low, impossible.
http://en.wikipedia.org/wiki/Ground_effect_in_aircraft