Karlie here from the Shopify Guru Team! When social media images don't update, it's usually because of the Open Graph tags that some themes for Shopify use to give social media platforms information about your website. Social media platforms like Facebook, Twitter and other services use Open Graph. To refresh your social media information and images once you've updated them in your storefront, you can simply use the Facebook Sharing Debugger, here, to update your stores imagery on social media.
I hope this has answered your question. If not, or if you have any further questions please let me know. I will be happy to help!