His argument is literally just "I don't know who these people are, but they're white, and don't look like they belong in this picture, so I think it's a photoshop".
His comment was posted without knowledge of who the subjects of the photo were, and with the specific "white gangsta from da hood" racial epithet. His comment was also not about how genuine they look, hanging out with soldiers, but just how it appears to him to be a total fabrication. Obviously the IC guys are not hardened killers, though they are arguably journalists who saw some abhorrent things- that much is undeniable. Just because they are showing solidarity with a militia group by holding AKs and probably giving a speech or similar, does not mean that they specifically need to be killing people to prove they were actually there.
I mean, Invisible Children is a terrible organization with shady, ideologically corrupt goals for so many reasons that you don't have to say their photos with real live black people are photoshopped. It just strikes me as a ridiculously low attack to make on them, for reasons that Delta might not have intended or realized.
Edit: I'm respectfully going to mostly ignore the repeated instance of attempting to dismiss their appearance as trying to be 'gangsta' because it's such a racially loaded term that you should really consider why you're using that instead of just 'trying to look tough'.