These are the optimal sizes for photos you upload to Facebook. I had no idea. I knew that FB automatically resizes and formats your photos, but did not know these were the magic numbers. Also:
- 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, select the High Quality option when you upload
To avoid compression when you upload your cover photo (emphasis mine), make sure the file size is less than 100KB - Save your image as a JPEG with an sRGB color profile
I copied the entire contents here, but the FB help link is at: https://www.facebook.com/help/266520536764594