To load the image descriptor from a file in the plugin use:
final URL fullPathString = FileLocator.find(bundle, new Path("icons/stop.png"), null);
ImageDescriptor imgDesc = ImageDescriptor.createFromURL(fullPathString);
where bundle
is your plugin bundle, get this from your Activator or:
Bundle bundle = Platform.getBundle("plugin id");
Don't forget to add your icons
folder to the build.properties
file so that it is included in the built plugin.