Convert .png image icon to .icns file (MacOS)
You can achieve this using the sips command.
First, store your icon as follows:

In an image file of size 1024 x 1024 pixels
In png format
In a file named Icon1024.png

Then execute the following commands
mkdir MyIcon.iconset
sips -z 16 16     Icon1024.png --out MyIcon.iconset/icon_16x16.png
sips -z 32 32     Icon1024.png --out MyIcon.iconset/icon_16x16@2x.png
sips -z 32 32     Icon1024.png --out MyIcon.iconset/icon_32x32.png
sips -z 64 64     Icon1024.png --out MyIcon.iconset/icon_32x32@2x.png
sips -z 128 128   Icon1024.png --out MyIcon.iconset/icon_128x128.png
sips -z 256 256   Icon1024.png --out MyIcon.iconset/icon_128x128@2x.png
sips -z 256 256   Icon1024.png --out MyIcon.iconset/icon_256x256.png
sips -z 512 512   Icon1024.png --out MyIcon.iconset/icon_256x256@2x.png
sips -z 512 512   Icon1024.png --out MyIcon.iconset/icon_512x512.png
cp Icon1024.png MyIcon.iconset/icon_512x512@2x.png
iconutil -c icns MyIcon.iconset
rm -R MyIcon.iconset

The result will be a file named MyIcon.icns that you can use to add to your .app directory.
Source: https://stackoverflow.com/questions/12306223/how-to-manually-create-icns-files-using-iconutil