Icon Glue is probably the easiest way to quickly create Apple Icon Images (.icns) or Xcode asset catalogs for your macOS and iOS apps from a single 1024x1024 pixels sized PNG or TIFF image.
You can also create all iOS app icons you need for submitting apps to the App Store in just one go.
Just open your image, select what you want to create (Icon Glue will remember these settings) and save it. You can also open an macOS 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, .iconset image sets and Xcode asset catalogs from a single PNG file|
|✓||create the optional iTunesArtwork.png images|
|✓||create all app icons required for your iOS apps|
|✓||create all icons required for the Apple Watch|
|✓||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.