You can include the splash screen image in a jar without using launch4J. The java tutorial says:

How to Use a JAR File to Display Splash Screen

If your application is packaged in a JAR file, you can use the SplashScreen-Image option in a manifest file to show a splash screen. Place the image in the JAR file and specify the path in the option as follows:
Manifest-Version: 1.0
Main-Class: <class name>
SplashScreen-Image: <image name>