What is the "Social Media Default Image"? Look here for the answer!
Social Media Default Image
The social media default image is what is used sometimes when you post something to a social media website and for whatever reason, they can't get access to the preview image that shows up. When that happens, it will use that social media default posting image instead of showing a blank image. That USUALLY only happens when using a content banner.
At the same time, when you create a content banner, you'll see a similar option on there as well. What will happen in that case is if you upload an image to that on the banner, it will use that image instead. So the one under "Website" > "Configuration" is more of a backup.
As for the type/size of the image, you'll want to upload something shaped 1200 x 628 pixels or the minimum suggested size of 600 x 314 pixels.
Here is the basic shape you want it to be:
