You're right - those pics have nearly 2:1 aspect ratio! (In XNview, if you drag the mouse over the image from top left to bottom right (i.e. select the whole image), it prints the width, height and aspect ratio in the status bar at the bottom - quite handy!)
The resolution for a 16x9 aspect ratio image that's 480 pixels high is the rather awkward 853.3333 x 480, which some quick sums indicate is 1.185 * 720 - so 1.3333 is definitely not right - sorry for the misinformation! NTSC doesn't have a square pixel aspect ratio, so it's not a 4:3 -> 16:9 transform.
I usually use fixed pixel sizes, e.g.
16x9 sizes: 1024 x 576, 853 x 480, 800 x 450, 720 x 405
4x3 sizes: 768 x 576, 720 x 540, 640 x 480
I have scripts set up for most of those sizes so I just load the specific one I want, rather than using a ratio
