How do i add an icon to the context menu?

This is not recommended for NB RCP (it will break HMI standards on Mac OS X for example), but is still possible.

Steps:

let your action implement Presenter.Popup and return a JMenuItem instance for the current action (see [1], [2])
set the icon via putValue(javax.swing.Action.SMALL_ICON, …)
set the icon via putValue("iconBase", …) to make it work properly in the toolbar. Otherwise you will get a fixed size icon.
By using setEnabled, it makes sure every listener will be notified (JButton, JMenuItem…). Moreover it is safer to call it on the EDT since it is not sure which thread triggered the event in the first place.

@ActionID(
        category = "Build",
        id = "sample.contextmenu.HelloIconAction")
@ActionReferences({
    @ActionReference(path = "Menu/File", position = 0),
    @ActionReference(path = "Loaders/Languages/Actions", position = 0),
    @ActionReference(path="Projects/Actions")
})
@ActionRegistration(
        displayName = "#CTL_HelloIconAction")

@Messages("CTL_HelloIconAction=Hello Icon Action")
public final class HelloIconAction extends AbstractAction implements Presenter.Popup {

    @StaticResource
    private static final String ICON = "sample/contextmenu/sample.gif";
    private static final long serialVersionUID = 1L;
    private final LookupListener lookupListener;
    private final Lookup.Result<Project> result;

    public HelloIconAction() {
        putValue(SMALL_ICON, ImageUtilities.loadImageIcon(ICON, false));
        putValue(NAME, Bundle.CTL_HelloIconAction());
        putValue("iconBase", ICON);
        result = Utilities.actionsGlobalContext().lookupResult(Project.class);
        this.lookupListener = new LookupListener() {

            @Override
            public void resultChanged(LookupEvent ev) {
                EventQueue.invokeLater(new Runnable() {

                    @Override
                    public void run() {
                        setEnabled(!result.allInstances().isEmpty());
                    }
                });
            }
        };
        result.addLookupListener(WeakListeners.create(LookupListener.class, this.lookupListener, result));
        this.lookupListener.resultChanged(null);
    }

    @Override
    public void actionPerformed(ActionEvent ev) {
        for (Project project : result.allInstances()) {
           JOptionPane.showMessageDialog(null, "Hello colorful project.\n" + project.toString());
        }
    }

    @Override
    public JMenuItem getPopupPresenter() {
        return new JMenuItem(this);
    }
}

Apache NetBeans 23

How do i add an icon to the context menu?