Guide
How to send iPhone photos to Android, or by email
The photo left your iPhone fine and arrived as a file the other person cannot open. That is HEIC, and there is one route that works every time.
Why it happens
Since iOS 11 the iPhone saves photos as HEIC, which stores roughly the same picture in about half the space. On the phone that is a straight win. It stops being one the moment the file goes somewhere that has never heard of the format — and support elsewhere is patchy rather than absent, which is the part that makes it confusing.
Modern Android can usually display a HEIC in its own photo viewer. What fails is everything after that: an older phone, an email client, a photo printing site, a job application form, a messaging app that passes the file through untouched. The recipient does not get a helpful error. They get a file that will not open.
The route that always works: convert first
Turn the photos into JPG before you send them. JPG has been universal since the 1990s; there is no device, app, or upload form that refuses it. You can do this on the iPhone itself — here is how — so it does not need a computer.
This is the only option that fixes the file rather than the recipient's device. Everything else asks the other person to change something, which is a poor thing to ask when you just want them to see a photo.
Why "just email it" sometimes works and sometimes does not
iOS occasionally converts a photo to JPG on the way out, depending on the app and on how the photo was attached. When it does, everything is fine and the problem never appears. When it does not, the recipient gets a HEIC. Neither outcome is announced, and you cannot tell which happened by looking at your own sent folder.
That inconsistency is the real reason this is worth fixing at the file rather than hoping. Some messaging apps re-compress every photo to JPG as a side effect of shrinking it, which is why one app seems to "work" and another does not.
If you send photos often
Open Settings → Camera → Formats on the iPhone and choose Most Compatible. The camera then saves JPG from the start and the problem stops recurring. Photos already in your library are unaffected, so convert those once and you are done. What you give up is the storage saving — the trade-off, in full.