Since iOS 11, iPhones store photos as HEIC. The files are about half the size of JPG, but plenty of software still cannot open them.
Stop it happening in future
On the iPhone, Settings → Camera → Formats → “Most Compatible” makes the camera save JPG instead. Existing photos stay HEIC, so convert those.
Privacy
Photos often contain location data and private moments. The conversion runs on your device; the photos are not uploaded.