Customize Applescript app icon

• Basically you need an .icns icon file.
• Open your application with right-click > Show Package Contents
• Navigate to Contents > Resources
• Delete applet.icns
• Drag the custom icon file into the Resources folder
• Rename the icon file in Resources to applet.icns
• To update the icon appearance open and re-save the applet in Script Editor.

This is how I do it:

1. In Finder, select your icon file.
2. Get Info (CMD-I) on the file.
3. Click the icon in the top left corner of the Get Info window.
4. Copy it via CMD-C.
5. Now Get Info on your AppleScript file.
6. Click the icon in the top left corner of the Get Info window.
7. Paste the icon using CMD-V.

The AppleScript icon should be replaced by the icon copied from the icon file.

Just building into @pipwerks answers: You can also use Drag&Drop!

• (Optional) Create your icon online from any image on https://iconverticons.com/online/ and download de .icns file for your icon.
• Get Info (CMD-I) on you AppleScript file.
• Now Drag&Drop your .icns file into the small icon in the top left corner.

It works and now it's ready to add it to the dock or wherever you want.