It will depends on the format of the container where you will add the parallax. For example if you have a squared container (normally a div with col-lg-6) your images will fit properly on the container since both has the same aspect ratio.
If your container will have a wide aspect ratio (like a full width section), those images will not fit properly. Our suggestion on this case is use a Image Software Editor like Photoshop or any other to resize the image for a wide aspect ratio.
The parallax has some control options that also can help you: