Which browser are you using on iPhone? Can you use incognito mode to view the store? Also, make sure you have a fast internet connection by visiting fast.com.
The image on the banner does not open in any iPhone devices using Google Chrome or Safari, but as in your image below in my Online Store editor the mobile version shows the image correctly.
Could you please advise if it has anything to do with the CSS Code?
Let me know if you need further details to solve this.
Please let me know once you had the chance to check the website and if you detected any issue and how can we solve the image banner to be seen in iPhone devices.