Java SE (Java Platform, Standard Edition)

I have an application (with imageicons) jarred.
The application at first didn't load the images. After reading in topics here I understanded that this should be the right implementation for reading image icons out of jars:

unDoButton = new JButton(new ImageIcon(this.getClass().getClassLoader().getResource("img/vorige.jpg")));
		reDoButton = new JButton(new ImageIcon(this.getClass().getClassLoader().getResource("img/volgende.jpg")));
		treeViewButton = new JButton(new ImageIcon(this.getClass().getClassLoader().getResource("img/mappen.jpg")));
		weergaveButton = new JButton(new ImageIcon(this.getClass().getClassLoader().getResource("img/weergave.jpg")));
		bovenLiggendeMapButton = new JButton(new ImageIcon(this.getClass().getClassLoader().getResource("img/bovenLiggendeMap.jpg")));

After jarring the application, and running the jar file, nothings happens.
Changing it back to:

unDoButton = new JButton(new ImageIcon("img/vorige.jpg"));

..the application works, only the icons aren't loaded.

Anyone suggestions?

Greets,
Sjoerd