Why your iPhone photos will not open
In 2017 Apple switched the iPhone camera from JPG to HEIC, a format built on the same compression as modern video. It stores a photo in roughly half the space at the same visual quality, which is why your phone holds so many more pictures than it used to.
The catch appears the moment the photo leaves the Apple world. macOS and iOS decode HEIC at the operating system level, so everything on your phone and Mac just works. Windows does not, unless you buy Microsoft's HEIF Image Extensions from the Microsoft Store. Android support is inconsistent. Plenty of upload forms reject the file outright.
So the photo that looks perfectly normal on the phone that took it becomes unopenable the second you email it to a colleague. Converting to JPG removes the problem for good, because JPG has been readable by essentially every device since the 1990s.