HEIC files from an iPhone or another camera do not open in every browser environment. This site uses the image decoding available on your device. It does not upload the file to a conversion service and does not include a remote HEIC decoder. Try one photo before selecting a whole collection, especially when working on a computer that has not opened your phone photos before.
If decoding fails, changing the extension to JPG will not help. Open the original in your phone's photo application and export or share a JPEG copy through a supported workflow. You can also try a compatible Safari device. Support can vary with software and the particular file, so a successful conversion on one device does not prove every HEIC will work everywhere. Test the actual source file on the device you plan to use for the remaining photos.