For those who asked, here’s a simple Photoshop template for Facebook cover photos which will work on desktop and mobile, and should reduce the distortion Facebook puts into images by compressing or stretching them.
Cover photos are now 820×312 pixels. The image gets stretched and cropped and displays as 640×360 on smartphones.
So make sure your cover photo is taller than 360px and be sure the middle 640×312 of your cover photo contains what you need. Best size to upload is 1640×624 – then when Facebook compresses them they still stay sharp. But try to keep the file size under 50k. Yes, that’s hard, but it’s better for you to control the compression than to have Facebook’s auto-compression mess up your sharpness and colours.
The usual disclaimers apply – I can’t provide any support for this, and Facebook is constantly changing image sizes, so this may not be valid in the future.