Icon Glue is probably the easiest way to quickly create an Apple Icon Image (.icns) for your mac app from a single 1024x1024 pixels sized PNG or TIFF image.
Just open your image, select what you want to create (Icon Glue will remember these settings) and save it. You can also open an OS X application bundle to easily extract its icon without having to browse through the bundle's internal folder structure.
And if you should need just a few of them: the integrated iOS and OS X icon previews allow you to create these icons individually with a simple drag'n'drop gesture.
With Icon Glue you can easily ...
|✓||create .icns files or .iconset image sets from a single PNG file|
|✓||create the iTunesArtwork.png images required for app submissions on the Mac App Store|
|✓||create all app icons for your iOS apps|
|✓||extract all image representations from an existing .icns file|
|✓||extract an application bundle's icon and export it as .iconset and/or PNG file|
|✓||show previews of all iOS and OS X Icons to be created and allows you to save them one at a time|
Rectangular (non-square) or too small images will be adjusted accordingly to ensure that the result is always a valid, retina-compatible .icns image file.