bluesheart, check, if the iPhone is taking photos in HEIF format. This is described here.
Using HEIF or HEVC media on Apple devices - Apple Support
A Mac with macOS 10.11.6 El Capitan cannot process media in the new, space saving HEIF format. You will just be seeing an exclamation point.
You may want to change the Settings on the iPhone to take the photos in a more compatible format:
- In iOS 11, 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.
- 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.