Using images in JAR archives

I've recently tried to export my current project as a JAR file.
However, whenever I now try to start it, I am getting a
NullpointerException from where the pictures are supposed to be.

Below's some code showing the problem. Iam using Eclipse for the
export. I already tried refreshing and using several versions of
getResource(), getSystemResource() and so on using relatie or absolute
paths. I even tried a version using an InputStream, the result is
always the same:

Exception in thread "main" java.lang.NullPointerException
        at javax.swing.ImageIcon.<init>(ImageIcon.java:138>
        at ShowImages.loadImage(ShowImages.java:25)
        at ShowImages.<init>(ShowImages.java:14)
        at ShowImages.main(ShowImages.java:31)

In Eclipse, the project works just fine. The pictures are placed in a
folder "images" in the JAR archive, as it should be. The manifest
reads:

Manifest-Version: 1.0
Main-Class: ShowImages

Do you have an idea on how to get this to work? If so, could you give
me an explanation on how you did it, using which options/commands?

import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;

@SuppressWarnings("serial")
public class ShowImages extends JFrame {

    ShowImages(){
        this.setSize(500, 500);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        ImageIcon image1 = this.loadImage("images/image1.gif");
        JLabel label = new JLabel(image1);

        getContentPane().add(label);
        setVisible(true);
    }

    private ImageIcon loadImage(String path){
        java.net.URL url = ShowImages.class.getResource(path);
        ImageIcon titelbild = new ImageIcon(url);
        return titelbild;
    }

    public static void main(String[] args){
        ShowImages frame = new ShowImages();
    }
}