im not sure if u r actually using image plane feature or the way the tutorial here teachs u.

since they all same size, u can try using image place feature in maya instead of using polygon plane as told in tutorial.

(the top/side/front pictures of the airplane must be matching size, and canvas height must be same too)

1) open up maya. make it show top/front/side view. Don't move the camera at all.

2) then, from the top view, select View>Image Plane>Import Image.

3) browse and select the top view picture of the airplane

4) go View>Image Plane>Image plane attributes. Change center y to like.. 20

5) do same w/ side and front. Side (center x to -20) Front (center z to -20)

center thingies moves the image plane. i hope u get it workin. im new to maya too