I think it is best to put your pictures inside a package. This way, everything will be packaged and loaded from your JAR file.
Afterward you will have to load your images using the getResource(...) method from an instance of Class.
You will have something like this:
GameOfLife
|- src
| |- my
| | |- company
| | | |- app
| | | | | Board.java
| | | | | Frame.java
| | | | |- resources
| | | | | |- playToolbar.png
| | | | | |- pauseToolbar.png
Then to create an ImageIcon of "playToolbar.png" from the Frame class you will have to use:
new ImageIcon(Frame.class.getResource("resources/playToolbar.png"));
Or:
new ImageIcon(Frame.class.getResource("/my/company/app/resources/playToolbar.png"));
If you are using Netbeans GUI builder, it can load resources from package without any problem.
To autorun your Frame class you have to create a MANIFEST.MF file and put it inside a folder named META-INF at the root of your JAR. Inside, you can define Frame as the Main-Class:
Manifest-Version: 1.0
Main-Class: my.company.app.Frame
Eclipse can do this step automatically if you select export->Runnable Jar.