Sunny here from the Shopify Guru team and I would be happy to help with updating your social media sharing images.
This is a super common question and, most of the time, it can be an easy and quick fix. At first glance, it looks like you are using the Simple theme, which is one of our free theme templates and that's great because as part of all free Shopify themes, there is an area within your theme customizer where you can upload a social image that will be used when sharing your shop on social media. You can choose to upload a logo, product image or just about anything!
To access this area, simply head over to Online Store > Themes > Customize > Theme Settings > Social Media > Select image. Feel free to watch this quick video demonstration if you're a more visual learner. Once you've uploaded a new image in that area, we will need to use the Facebook Debugger to refresh and update your image from the internet. You'll just need to enter your website URL, click "Debug" and then hit the "Scrape Again" button, which looks like this:
You may have to click "Scrape Again" a few times to eventually see your new image appear and once you do, go ahead and share your shop link on social media to see if that worked. Let me know if you have any questions at all or run into issues along the way.