public class AlertWithIcon

extends Application {

    @Override

    public void start(Stage stage) {

        Alert alert = new Alert(Alert.AlertType.CONFIRMATION,

            "Are you sure you want to delete this item?",

        ButtonType.YES, ButtonType.NO);    

        alert.setHeaderText("Delete Item"); 

        ((Stage)alert.getDialogPane().getScene().getWindow()).getIcons().add(new image("GenericApp.png"));

        alert.showAndWait();

    }

}