Why iPhone photos will not open
Newer iPhones save photos as HEIC, a format that stores great quality in a small file. The downside is compatibility: many Windows programs, websites, and older apps cannot open HEIC, so the photo appears broken or refuses to upload. Converting to JPG fixes it instantly.
How to convert
Open a HEIC-to-JPG tool, choose your photos, and download them as JPG. A browser-based converter does the work on your device, so your photos are never uploaded — important for personal pictures. You can usually convert a whole batch at once.
Will quality drop?
JPG is a lossy format, so a tiny amount of detail is re-compressed, but at high quality the difference is invisible for normal viewing and sharing. If you need a lossless copy, convert to PNG instead — the files are larger but pixel-perfect.
Stop it happening again
On iPhone you can set the camera to capture in the more-compatible JPEG format (Settings › Camera › Formats › Most Compatible). New photos will then be JPG; existing HEIC photos still need converting.