Best Practices for Sharing
When customers share your landing page on Facebook your logo is used. Facebook uses a standard aspect ratio of 1.91:1 when displaying your logo. The recommended logo size is 600px X 315px. Your logo should not be smaller than 200px X 200px; Logo images that surpass the 1.91:1 ratio will be cropped or not display in the shared box. If no image displays then the share box will contain the store URL, title, and description.
There is an issue when your store is first shared after updating the logo. The first person to share your store after updating your logo might not see an image or see a previous logo instead. To prevent this after adding or updating your logo, log into Facebook and use the URL Debugger