Why iPhone photos need converting
Since iOS 11, iPhones save photos as HEIC by default because it takes about half the space of JPG. The problem is that many Windows computers, Android phones, older photo editors, printers and upload forms cannot open HEIC. Converting to JPG makes your photos work almost everywhere without changing your iPhone settings.
What happens when you convert
Each HEIC photo is decoded in your browser and saved as a new JPG. The Quality slider (40–100, default 92) decides how much the JPG is compressed: 92 looks the same as the original for almost every photo. The optional width box shrinks large photos — it never enlarges them — and the height follows automatically. Many photos can be converted together and downloaded in one ZIP, or one by one from the file list.
What is and is not kept
The picture, its colours and its size (unless you set a width) are kept. Hidden photo details such as GPS location, date and camera model are not copied, which also protects your privacy when sharing. Live Photo motion and portrait depth effects are not carried over — you get a normal still picture.
Common problems and how to fix them
The JPG is bigger than the HEIC: that is normal, because HEIC compresses better; lower the quality to 80–85 if size matters. A form still rejects the photo because of its KB size: use Resize Photo to 100KB or Resize Image to 50KB. A photo appears sideways: rotate it with Rotate Image. You need transparency or lossless quality: use HEIC to PNG.
Good habits
Keep your original HEIC photos as the master copies, convert only the photos you need to share, and on iPhone you can set Settings → Camera → Formats → Most Compatible if you always want JPG.