You really need to work on your posing before even opening GIMP, photoshop or whatever. If you edit before getting the basics down you'll just be polishing a pile of shit. Sure, it might look fancy, but it's still a piece of shit.
The muzzleflashes are really too yellow, the posing is downright horrible and it lacks any interesting lighting.
The guy running is too stiff and shooting with 1 hand? (horrible accuracy), dead guy seems like you flipped him there,why are you using suppressors on a mech?, The guy in-front elbow is way too bent,
The effects lack depth, and you gotta STOP DROP AND http://www.facepunch.com/showthread.php?t=203222