You can use the Social Preview feature to choose the image displayed when sharing your site via Facebook and LinkedIn.
From the editor, click the 'Admin' tab.
On the left sidebar, select 'Social Preview'.
Click 'Select Image' and choose an image from your file manager, or upload a new one. TIP: You could take a screenshot of your site design and use this as the thumbnail. NB: Minimum size should be 200x200px, with a maximum of 1200x630px.
Click 'Use this' and the image will appear in the Social Preview area.
You can add more than one image here, which will enable you to choose which image you'd like to use when sharing on Facebook. The first image you add will appear first in the list, and will be the default share image. Note that you cannot change the order of these once added, however you can delete individual images by clicking the trash can icon next to the image.