Sorry to say but, you cannot change the icon displayed on the jar file directly. That's the icon displayed by your file browser/operating system for the file type.
You cannot do that unless:
1. You change the icon in the registry (for windows) or icon theme(for POSIX). But this will change the icon for all jars.
2. Use a shell hook. But it is Windows only and requires messy C++ coding.
These are not good practices. They just mess up things.
If your client wants to see the icon everywhere, then you can try these:
1. Add a shortcut to the desktop/app launcher with your icon.
2. If your deployment platform is windows
wrap your app in a launcher exe with your icon (ico file). Launch4j
is a good option.
If platform is Linux, then add a .desktop file refering the icon and the icon (png file) to /usr/share/icons/hicolor/SIZE/apps/ where size must be 16,24,32,64 and 256
Sorry, I don't know how to assign an app icon on that platform, a bit googling will do.