Social cards enables web page to become a rich object in a social media. In other words when you share your website link on social media it gets displayed as rich media objects.
Please follow these steps if you want to change image for your website's social card
Login as admin > Open site builder > click on More (Top Left Corner) > Click On Settings > Click on Title & Description tab> Click on Upload banner image under Social share banner > Click on update
Note : Once you have loaded new image, you need to clear the cache from Facebook crawler. Steps to clear Facebook crawler cache :
Click on Debug button
Click "Scrape Again" button 2-3 times