There are plenty of articles about how to adjust images for uploading to Facebook. But I just found one on a forum with a succinct response which appears to come from a Facebook employee. I also remember a previous article that suggest a JPG compression of 6 or less. Something to do with the colour compression. But here an important factor is also the maximum size of 100kb.
“We automatically take care of resizing and formatting your photos for you when you add them to Facebook. If you’d like to use more advanced techniques to make your photos look better, try these tips:
Resize your photo before you post it. These are the sizes (width) we support:
Regular photos 720 px, 960 px, 2048 px
Cover photos 851 px by 315 px
If you use a 2048 px photo, make sure to select the High Quality option when you upload it.
To avoid compression when you upload your cover photo, make sure the file size is less than 100 KB.
Save your image as a JPEG with an sRGB color profile.
Posted about a month ago by Binny Rutherford”