Dallas here from the Shopify Social Care Team. That is a great question and I'm more than happy to lend a hand.
In order to change the social sharing image you just have to go into your theme customizer (Online Store > Themes > Customize)
— and click under Theme Settings > Social Media
— and then change or add a particular Social Sharing Image.
Once you do that should be the image that shows up when your store URL is shared on social media.
This link has a bit more information, but if you have any more questions at all please let me know!