Perhaps your screen is misadjusted.
Take this image and measure if the circle is actually a circle or an oval:
Use a real ruler for measuring, not a software tool.
4/3:
(recompressed = too heavy and not very good quality, but it is just for the shape of the picture)
It would also be better without black border
To do the test take a car seen from side (strict side, i.e. no front or rear part visible), and check the wheels. Another test is to find a picture in a non-4/3 movie (16/9 or cinemascope).
Maybe that the picture that I resized to 4/3 is wrong due to the black borders (I do not know if it is 4/3 with or without the borders - I guess it should be without), but for me it seems to have a better shape than the original one.