icns Tool is a great tool that helps mac app developers to convert one PNG file (you only need to provide one 1024px X 1024px PNG file) to .icns file that used by your Mac App. And it can also generate document type .icns files for your Document-Based applications(you only need to provide one 1024px X 1024px PNG file and one background PNG file).
The generated .icns file consists of the following 10 files:
icon_16x16.png, icon_16x16@2x.png
icon_32x32.png, icon_32x32@2x.png
icon_128x128.png, icon_128x128@2x.png
icon_256x256.png, icon_256x256@2x.png
icon_512x512.png, icon_512x512@2x.png

✔ Feature 1: 

Generate .icns file from the given PNG file. 

• Usage:

1. Browse a PNG file (Recommended size 1024x1024) for App Icon File field.
2. Browse a folder for Output Dir field.

3. Click Generate button.

✔ Feature 2: 
    
Generate document types .icns files from the given PNG file and extensions.

• Usage:
    
1. Drag and drop a background and app icon PNG file to the Background and App Icon image views.
2. Input some extensions.
3 Browse a folder for Output Dir.
4. Click Generate button.