Which iMac model do you have? Several users reported, that Mojave on their iMac has a bug and cannot handle the HEIC format. The iMac models in question are using the same graphics card, all older Macs from 2013 or around that time. This bug will hopefully be fixed by the next update to Mojave.
You can change the settings on your iPhone, so that it will take all new photo in a more compatible format. This is explained here: Using HEIF or HEVC media on Apple devices - Apple Support
Though capturing in HEIF and HEVC format is recommended, you can set these devices to capture media using older formats that are more broadly compatible with other operating systems and devices:
- 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.
And to download HeIF media from your iPhone to a Mac by USB as JPEGS change this setting:
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 when importing.
