From what I am reading online, the Tools -> Adjust Size menu selection will allow you to scale your picture to any desired number of horizontal and vertical pixels. The picture will distort (round circles will appear elliptical, for example) if you scale by different percentages in the horizontal and vertical directions. None of the overall view of the picture is lost when the size is adjusted.
If you want your photo to always appear in its entirety, no matter if viewed on a desktop or mobile device, you will need to live with the fact that you need to add areas of border pixels to the photo so that you may end up with the border pixels showing on both sides when viewed on a desktop and top and bottom when viewed on a mobile device. This way, the only thing that Facebook will be cropping is the border pixels and not the actual picture portion. I don't know if the Mac Preview application allows one to easily add such border pixels.
If you prefer, upload your original jpeg picture file in your reply (there should be an Upload a File button below your post's text entry box, tell me the color you wish for the border pixels, and also tell me if you want the photo to be uncropped in both the desktop and mobile views or just either the mobile or desktop view. I will add the border for you and upload the results.