This
op-ed in the NY Times explains how when Ali Shalal Qaissi claimed to be the hooded man standing on the box, and told his story to the press, when it was later discovered he was not the man, his story became attached to that picture in the publics mind, anyway.
It also talks about how Qaissi has a deformed hand (rifle blew up at a wedding), that the hooded man does not have, but people looking at the picture saw what they wanted to see.
It's an interesting observation on the power and shortcomings of photos in the press that's worth reading.