HEIC (High Efficiency Image Container) is Apple's default photo format used on iPhone and iPad. It offers better compression than JPEG at the same quality, but has limited compatibility outside Apple devices.

Does converting HEIC to ICO preserve the full image quality?

Yes. The tool decodes the HEIC file at full resolution before downscaling to each ICO size. Use a source photo of at least 512×512 pixels for the sharpest results.