When I create ImageIcon class objects I use the following code:
iconX = new ImageIcon (getClass().getResource(\"imageX.png\"))
Simply supply the path to the resource.
So, if you put the image in "/resources/images" within your Jar, you would simply use
iconX = new ImageIcon(getClass().getResource("/resources/images/imageX.png"))
Essentially what you're saying is, class loader, please search your class path for the following resource.