Facebook automatically resizes and compresses your image. I haven’t been able to upload an image which ended up looking exactly the same as the high quality image that I uploaded. Below are however some steps to aid in getting some of that quality back into your image.
Some help from Facebook? (I found this in their help files). https://www.facebook.com/help/266520536764594
How can I make sure that my photos display in the highest possible quality?
We automatically take care of resizing and formatting your photos for you. If you'd like to use more advanced techniques to make your photos look better, try these tips:
§ Resize your photo before you upload it. These are the sizes we support:
720 px, 960 px, 2048 px
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 under 100 KB.
§ Save your image as a JPEG with an sRGB color profile.
Now how do you go about doing the resizing?
In your photo editing program (I use Photoshop but there are many other programs you can use, even Microsoft Paint) you can resize your image.
Make sure “constrain proportions” is checked if there is such an option otherwise your image may end up looking really strange.
Now set your long edge (width or height, whichever is greater) to 2048 pixels.
Let the other number change on its own to preserve the aspect ratio of your photo.
Save your image as a JPEG with an sRGB color profile
Hope this helps!