Can you tell us how you run your code (e.g. command line, if so, what is the classpath), and where all the locations of splash.png are in your folder structure?
The file needs to be on the classpath, as that appears to be from where this code loads the image.
String iconPath = sourceSettings.getSettingsDialogImage();
if(iconPath == null){
iconPath = "";
}
final URL iconUrl = JmeSystem.class.getResource(iconPath.startsWith("/") ? iconPath : "/" + iconPath);
if (iconUrl == null) {
// *****LINE 112 below*****
throw new AssetNotFoundException(sourceSettings.getSettingsDialogImage());
}