Only saw the story this morning! I believe the piece I read said that a group called "The Mujahideen Squadron" made the claim. Personally not heard of them.
Is this a sign that they are getting desperate? How long did they think they would have been able to run with that story? Did they think the seps would 'just suddenly think' that one of its troops were missing without checking first?
I am a little cynical; it seems to be an M16 in the background and that is not Arabic. I fear that Yahoo have fallen for US prank. I doubt that the picture was ever posted on an Islamists website.
Since IGS block access to source material (Islamist websites), if want to see Islamic propaganda you should visit rightwing US bloggers sites. They lift the footage before they are blocked or found by the mainstream media strange but IGS does not seem worried about their contents! Having watched two beheading + a lot other footage it seems that IGS is not the ball.