Here's some examples of what I've done regarding doctoring a photo with just microsoft paint:
This is my doctor:
This is my proof:
This is my original:
I don't know much about the website you shared but I can't understand it unless someone like yourself wishes to explain what differential compression artifacts means etc. I did look at this website regarding the photo you shared and found lines in the wall where his bag could have been. Also his jacket leans to the right on his right shoulder where his bag was when he was carrying it. Here's a video of the bag on his shoulder:
Now look at the colour of his bag and compare it with the first video I put up. The bag with the bomb in it and the bag that this guy is wearing don't look anything like one another. The guys bag was white. The bag of the bomb was black.
I'm always open to hearing the truth so if you can present something more compelling that would be great. But even if this photo was not doctored it still could be hiding the bag on his shoulder which it definitely does since his jacket is not sitting on his right shoulder properly giving the idea that he's carrying his bag on that shoulder.