What HEIC is and why iPhones use it
HEIC is Apple's default photo format. It stores images using the HEVC video codec inside the HEIF container, which lets an iPhone keep a picture at the same visual quality as a JPG in about half the space. Photos taken on any iPhone or iPad since iOS 11 are saved this way unless you change the camera setting.
The trade-off is compatibility. Windows needs extra add-ons to preview HEIC, and many web forms, printers and older programs reject the file outright. Convert to JPG for the widest support, PNG for lossless editing, or WebP for web pages.
How the conversion works without an upload
Safari can open HEIC itself. Other browsers cannot, so the first time you drop a HEIC photo this page loads a small decoder that runs inside your browser, and the photo is decoded on your own device. Nothing is sent to a server, and the decoder is reused for every photo after that.
What to expect
Only the main photo is converted: the video of a Live Photo, depth data and any extra frames are left behind. Display P3 colors are mapped to standard sRGB, and capture details such as GPS location are not copied. Converting to HEIC is not offered, because no browser can write the format.