Agree that they're Marines, not ING.
When in Al-Anbar, we issued a few PPSh-41s to our local ING unit, as the officers wanted a "status" weapon to distinguish themselves from the enlisted. We were rather short on pistols at the time.
Regarding SMGs, non-issue gear, etc. Generally, when I see a picture of a Marine with an Iraqi weapon, I assume it's a photo op. I planned to do a cool "clearing a stairway" photo op with a 1938 Beretta submachinegun, but never got around to it. Would have been fun just for the novelty. (For the pics I did get, they're up at photos.yahoo.com/signpostmk under the "Iraqi Range" section). Every time we had a pile of Iraqi weapons, every lance corporal wants to grab one and strike a pose for the camera.
That said, I'm sure there are folks using non-standard gear. I knew a few Marines who carried AKs on missions, one who carried an Iranian G3 ripoff, and one who carried a 9mm Sterling subgun. Most of those were guys from smaller, specialized units (not Special Forces, just specialized support personnel) who weren't under particularly close scrutiny from higher, and could get away with more. In most cases, it was definitely a combination of "good weapon for the job" and "gee, isn't this cool", rather than being just one or the other.
So, odds are these guys are posing for fun, but if they're not, I'd say they picked a pretty interesting choice for room clearing.