Using Image Planes
If you want to make a composition of your render within an (existing) photo, you can use Image Planes in Maya to position your camera properly. Image planes are planes that are attached to the camera and display an image in the field of view of the camera.
If you use image planes for a photo composition, it's recommended that you set your Render Settings render resolution to the dimensions of the image your using for the image plane (or at least the same aspect ratio).
Creating an Image Plane
Open the Attribute editor for the camera you want to create an image plane for. Select the camera and open the Attribute editor, or select View > Camera Attribute editor in the viewport in which the camera is active.
Go to environment and click Create. Once you've created an image plane, you can click the box with the square and arrow (goto input connections) or click the imagePlane tab.
In the attributes of the created image plane, select the image you want to use. Make sure the resolution (pixel dimensions) of the image are sufficient. When you use image planes for photo compositions, use an high resolution image. If you don't use the image plane for rendering, at least make sure the low resolution image has the same aspect ratio.
You can also specify the distance between camera and image plane, called Depth, here. Make sure the value for depth is high enough, or your image plane will block the view on your geometry.
Positioning the camera with Image Plane
Now you can position your camera. When the camera is selected, you can move the Center of Interest using the Manipulator Tool.
Removing the Image Plane
Go to the Hypershade. Select the Cameras tab. Locate the imagePlane node and select it. Press [del] to delete it.