Customizing Link Thumbnails
To customize your link thumbnails switch the Custom Thumbnail toggle for the link on (shown below). Click the Upload button, then select the image you want to use in the file browser. Crop and zoom the image to the size you want, then hit Done.
Don't forget to save your links after making changes to them in the editor!
This feature is only available for the Entrepreneur Plan and above.
You can choose if you want your custom thumbnails to be square or round under the appearance tab in the Page Editor.