Sometimes when you upload a Photo onto Facebook you’ll notice a significant change in quality. Facebook recommends that you save your image as a JPEG with an sRGB color profile.
To change the color profile of an image to sRGB go to Edit > Convert To Profile…, in this new window, choose sRGB IEC61966-2.1 from the Profile drop-down. The result will be a closer representation to what you will see on Facebook. You can then save your image and upload it to Facebook.
Use these widths: 720 px, 960 px, 2048 px