Whatever you are promoting on Facebook, make the most of the space you have and use the correct dimensions on your pages:
Your cover image is a great space to show off your business so it’s important that you set the correct size – if you use an image that is too small it will be stretched , if you use one that is too large you will loose some of the image.
Cover photos are 851 pixels wide and 315 pixels tall and must be at least 399 pixels wide.
Share with your fans key points and milestone in your journey
Milestone and highlighted images are 843 pixels wide x 403 pixels tall.
Shared images / pinned posts are square images that fit perfectly within the timeline, if these images are not the optimal size, Facebook will only show part of the image.
Shared images are 403 pixels wide x 403 pixels tall.