To edit the Social Media SEO settings of a SEO template please follow these instructions:
Go to My Content >> SEO Templates
Search for the SEO template to be edited and click on customize or "Edit" if it's already a custom template:
On the SEO Settings tab and Scroll down until you find the area for Social Media Sharing. Edit the information accordingly:
It is possible to set an image as the "Social Media Shared Image", but like applying SEO settings, these are only suggestions and the social media sites and search engines will choose at their own discretion whether to use them or not.
You can also use this article if you wish to have a generic social media image:
How to Set a Default Social Media Image
Do not forget to Click on "Save this page".