HEIC is the format iPhones and iPads use for photos by default. It keeps files small, but plenty of places still don't open it: older Windows PCs, some Android apps, many websites and online forms, photo printing kiosks. Converting HEIC to JPG is the simplest fix, because JPG opens practically everywhere.
Drop the photos and download them as JPG. This page is already set up for it: high quality and the original size, so nothing is scaled down. The rotation saved by the camera is applied, so portrait shots don't come out sideways, and the colors stay as they were on your phone. A 12-megapixel photo takes about 3 seconds. You can drop a whole batch at once and download everything as one archive, which helps after a trip or when you're moving a year of photos to a PC.
The conversion runs in your browser. Your photos aren't uploaded to our server or anywhere else, which matters for pictures of family, documents or your home. It also means no waiting for uploads on a slow connection.
The JPG usually ends up a bit bigger than the HEIC. That's expected: HEIC compresses more efficiently, and the JPG is saved at high quality to keep the detail. If you need smaller files, for example to send by email or to fit a form's upload limit, use compress image afterwards, or pick medium quality and a smaller size in the image converter.
Two tips for the future. On an iPhone you can switch the camera to Most Compatible in Settings, under Camera and then Formats, so new photos are saved as JPG from the start. And when you email photos from the Photos app, iOS often converts them for you; the trouble usually starts when you copy files to a computer by cable or through the cloud.
If you need PNG or WebP rather than JPG, or want to change the size, the image converter has all the settings. For a product photo, the background remover gives you a clean cut-out.