Instead of "Brute-force" the image path try to figure out what's the path you try to load the image from. You can look it up with the following
System.out.println("Path: " + (this.getClass().getClassLoader().getResource("")).getPath());
afterwards try to load it like this or like @Balint Bako allready sed.
Image img;
URL url = new File("the path you found out").toURI().toURL();
BufferedImage img = ImageIO.read(url);