1. Why the browser cannot do it on Windows
Chrome, Edge and Firefox do not include a HEIC decoder, so online converters either upload your photos to a server or ship their own decoder. HEIC Studio does neither. If you have an iPhone, iPad or Mac at hand, open the converter in Safari there – it converts locally. On Windows, use one of the options below.
2. Photos app (Windows 11)
Install the HEIF Image Extensions and HEVC Video Extensions from the Microsoft Store (the HEVC extension may cost a small fee). Then open the photo in Photos, choose ⋯ → Save as and select JPG. This works one photo at a time.
3. Paint
With the same extensions installed, Paint can open HEIC files. Use File → Save as → JPEG picture. Again, one photo at a time, and Paint does not keep EXIF data such as the capture date.
4. Let the iPhone convert when copying
On the iPhone go to Settings → Photos and under Transfer to Mac or PC choose Automatic. Photos copied over a USB cable are then converted to JPG. To stop new photos being HEIC at all, choose Settings → Camera → Formats → Most Compatible.