MacOS 10.11 El Capitan does not support HEIC images. You need a Mac with macOS 10.13 work with HEIC. See this page on how to set the settings on your iPhone to have them converted to JPEG when you import them to a computer. Or set the iPhone to take the photos in a more compatible format:
Using HEIF or HEVC media on Apple devices - Apple Support
To convert the HEIC image you have already on your mac try the online converter at http://heictojpg.com/
To change the settings on the iPhone to capture JPEG instead of HEIC:
Though capturing in HEIF and HEVC format is recommended, you can set these devices to capture media using the older formats, which are more broadly compatible with other devices and operating systems:
- Go to Settings > Camera.
- Tap Formats.
- Tap Most Compatible. This setting is available only on devices that can capture media in HEIF or HEVC format, and only when using iOS 11 or later.
- All new photos and videos will now use JPEG or H.264 format. To return to using the space-saving HEIF and HEVC formats, choose High Efficiency.
To change the Settings to transfer the HEIC as JPEGS:
When you import HEIF or HEVC media from an attached iOS device to Photos, Image Capture, or a PC, the media might be converted to JPEG or H.264.
You can change this import behavior in iOS 11 or later. Go to Settings > Photos. In the TRANSFER TO MAC OR PC section, tap Keep Originals to prevent the media from being converted to JPEG or H.264 when importing.