Since iOS 11, iPhones save photos as HEIC by default. It's an excellent format — half the size of JPG at the same quality — but Windows only displays it after buying HEVC codec extensions, and many apps never open it at all. The fastest fix is converting the photos you need to JPG: drop them in the tool above and download copies that open on any PC, in any program, on any website.
To avoid the problem for future photos, you can set your iPhone to shoot JPG directly: Settings → Camera → Formats → « Most Compatible ». The trade-off is files roughly twice as large. Most people keep HEIC on the phone and convert on demand — more about the format on the HEIC to JPG page.