Updating the Facebook Cover Photo keeps the page looking fresh and triggers a notification in follower timelines. Timing them carefully with the season, with a specific event, new product or service encourages engagement, especially with an added comment or link. It’s a simple way to create new content on a regular basis.

As of March 2018 the specs are:

• 851 pixels wide by 315 pixels high for cover photos
• Save with a file size of less than 100KB
• Check the high quality box when you create an album in Facebook so that images maintain resolution and quality
• Upload as a JPG with an sRGB color profile

Example: 

Source:

• https://designshack.net/articles/graphics/facebook-cover-image-tips/