You can set dedicated graphics for adverts published on social media. It is possible to add one general graphic for all advertisements, as well as different for each advertisement template.
1) To add general graphics for all ads, click Settings-> Advertisement and Clauses-> Advertisement. There you will find the "Header image for social media" section, in which you will be able to upload the image.
2) To add different graphics for specific advert templates go to Settings-> Advertisement and Clauses-> Advertisement Templates. As above, you will find a dedicated section for loading an image on social media.