You can set a social sharing image for your store under Online Store > Preferences. The image you add under these settings should appear whenever you share your store link via text or any social media channel that populates a preview image.
If you’d rather set an image specifically for this page rather than the entire store, another option is to create a collection called “All” and update your “SHOP” link to direct customers to that specific collection page. Then, you’ll be able to set a featured image for the collection.
Let me know if one of these options works for you and if you need any help with either.