This article explains what an ICNS file is and how to open it on your computer. It also explains how to convert this file to another image format, such as PNG, ICO, etc.
How to Apply ICNS Icon Files to Folders in Mac OS X
A file with the ICNS file extension is a macOS icon resource file (often referred to as the Apple Icon Image Format) that macOS applications use to customize how their icons are displayed in the Finder and in the OS X Dock. They are equivalent in most respects to the ICO files used in Windows.
An application package typically stores the ICNS files in the /Contents/Resources/ folder and references the files in the application's Mac OS X Property List (.PLIST) file.
One or more images can be stored in the same icon file and are normally created from a PNG file. The format supports the following sizes: 16×16, 32×32, 48×48, 128×128, 256×256, 512×512 and 1024×1024 pixels.
