Using your HEIC formatted photos taken with your iPhone on your website will first need to be converted to an acceptable file format. We have found that https://heictojpg.com/ does a good job converting HEIC to JPEG, and it is free.
You might also want to consider changing the settings in your iPhone if it is using iOS11 or newer, so it no longer saves images as HEIC but as JPG. The following article by Gihosoft reviews where to find and how to change the appropriate settings.