a couple of ways, you can go to view in the viewport and at the bottom select import image,this will give a image plane attatched to the camera or you can create a polygon plane in the modeling menu then in the hypershade create a new Lambert then double click on the Lambet and double click the checked box next to color and choose file in the new window and then where it says image name click the folder icon and browse to your image.lastly middle mouse button drag the new lambert with your image to your plane.
This a more long winded and you will have to make your plane the same ratio as your image ie if the image is 600 x 1000 then make the plane 6 x 10 or 60 x 100.Ther are a few advantages to doing it the long way because you can see the image in all views where as when done on the camera you can only see it in that camera view,also it it easier to move, scale the image plane and swap out images if you need to in future.