GConvert handles all standard Macintosh icon images including recent ones in 32-bit ARGB format (with alpha channel): thumbnail 128×128, 256×256 (OS X Tiger 10.4) and 512×512 (Mac OS X Leopard).

When you open a Macintosh icon in GConvert, it exactly behaves like a Windows icon: this means you can view its icon images, edit it, convert it to other formats and of course add it to icon libraries.