How do I convert a .PNG to an .ICNS file?

See this thread.

Unable to covert to .ICNS in Preview - Apple Community

When saving via Preview be sure to hold down Option key after selecting Save, it will show expanded save options include .icns

Hi - I am using Mac OS Mojave - and Preview doesn't give me the .ICNS option. I have .HEIC; .JPEG; .JPEG-2000; .OpenEXR; .PDF; .PNG & .TIFF. Where to now? Regards

Provided that you are starting with a 512x512x300 dpi or 1024x1024x300 dpi PNG image, you can generate an iconset, and ICNS icon using the Python 3 program at this location. The article behind that link.

MacOS does not install Python 3 so unless you have done so on your own from a 64-bit macOS install of Python 3.7.3 (current) from Python.org, this code will bork with macOS Python 2 (2.7.10). I have modified it to work with the System Python, and that code is included below. Copy and paste into a programmer's editor that retains Python source code formatting.

You need to make the script executable and run it from the Terminal command-line:

chmod +x ./gen_iconset.py
./gen_iconset.py ./foo512.png

This will create a foo512.iconset directory and a foo512.icns image by running iconutil as a background process.

This code was tested on macOS High Sierra 10.13.6 (17G6030), and should, but may not be backwards compatible with the Python version that Apple shipped with Mavericks.

Hi Eric - thanks for your response. I have Adobe Acrobat Pro - but no option to save as .ICNS. However, I did find a program (actually free) on the web call CloudConvert. It wouldn't convert every png or jpg file I had - I had to translate the graphic to a particular pixel size (using Adobe Photoshop Elements - but there could be other programs that do the same thing) and CloudConvert happily did the rest.

Have you tried Adobe Reader?

Adobe Reader