Facebook suggests some things in order to have the best quality images on Facebook. But the fact remains that Facebook still compresses images. In order to have the best images you can, it is wise to follow some rules.
These are Facebooks recommendations to get your photos and images to display in the highest possible quality?
Resize your photo to one of the following supported sizes:
Regular photos: 720px, 960px, 2048px (width) Cover photos: 851px by 315px
If you use a 2048px photo, make sure to select the High Quality option when you upload it