Here's something you can try at home. First, set up a camera. Then, grab a futuristic rifle (use a pretend one if there aren't any lying about). Now turn left until you're at a right angle to the lens.
Good. Now get a friend to stand behind you facing in the opposite direction. Then get a third friend to stand slightly behind both of you off to the other side.
Finally, adopt your most dramatic aiming pose and pretend you're on an alien planet.
Congratulations, you are a Mass Effect 3 screenshot!