Every photo an iPhone has taken since 2017 is stored as HEIC unless you changed a setting. It is a good format — roughly half the size of a JPG at the same quality — but it is licensed, so Windows, Android and every browser except Safari refuse to open it. You share a photo, the other person sees a broken icon.

The one-step fix

Convert the HEIC to JPG. JPG opens on everything made in the last twenty-five years. Drop the file on Convert Files, leave the target on .jpg (it is the default for HEIC), press Convert. The first time in Chrome a small decoder is fetched; after that it is instant.

For a photo you plan to edit further, choose .png instead: it is lossless, so nothing is thrown away before your edits. For a photo going into a document, choose .docx and you get a Word file with the picture already placed.

Stopping it at the source

If you would rather the phone never made HEIC files at all: Settings › Camera › Formats › Most Compatible. Photos will take about twice the space, which is the trade-off.

What about quality?

HEIC to JPG is lossy to lossy, so keep the quality slider at 90% or above. At that setting the difference is invisible; the file is simply a bit larger than the HEIC was.