As Facebook API resizes images automatically
The best practise to keep good quality images on the site is to resize them properly
before uploading them on your facebook page.
here some suggestions on sizing images to upload on facebook page
Examples: (in pixels)
Anyway you can upload the images with sizes you like as there in NO real restriction.
But if you keep the proportion near to 6:7 or 7:6 they will certainly look good quality.