Как изменить кнопку изображения GTK в C

Question

Я много чего перепробовал, но не могу изменить изображение кнопкой. В моем случае я бы хотел изменить изображение кнопки, позволяющей перевести мое приложение в полноэкранный режим и наоборот. Я пытался использовать несколько функций в документации, а также на форумах, но, похоже, ничего не получалось. Есть ли у вас какие-либо решения, чтобы предложить мне? Спасибо за вашу помощь:)

Ниже последнее, что я попробовал, прежде чем положиться на вас:

#include <gtk/gtk.h>
#include <time.h>
#include <err.h>

typedef struct Application Application;
struct Application
{
    //Builder
    GtkBuilder      *builder;

    //Windows
    GtkWidget       *splashscreen;
    GtkWidget       *window_main;

    //Buttons
    GtkWidget       *button_close;
    GtkWidget       *button_fullscreen;

    //Image
    GtkWidget        *img_fullscreen;
    GtkWidget        *img_leave_fullscreen;

    //Label
    GtkLabel        *progresslabel;

    //ProgressBar
    GtkProgressBar  *progressbar;

    //Error
    GError          *error;

    //Data
    int             is_fullscreen;
};

//Doesn't work fine :( Impossible to change picture in the button
void change_fullscreen_mode(gpointer app_data){

    Application *app = app_data;

    /* Check if picture is fullscreen or not */
    if(app->is_fullscreen == 1)
    {
        // try to assign appropriate picture to the button
        gtk_button_set_image(GTK_BUTTON(app->button_fullscreen),
                            app->img_leave_fullscreen);
        //unfullscreen the window of course
        gtk_window_unfullscreen(GTK_WINDOW(app->window_main));
        //set the variable that stands for this state
        app->is_fullscreen = 0;
    }
    else
    {
        // try to assign appropriate picture to the button
        gtk_button_set_image(GTK_BUTTON(app->button_fullscreen),
                            app->img_fullscreen);
        //fullscreen the window of course
        gtk_window_fullscreen(GTK_WINDOW(app->window_main));
        //set the variable that stands for this state
        app->is_fullscreen = 1;
    }
}

int main(int argc, char *argv[])
{
    //Init gtk
    gtk_init(&argc, &argv);
    Application app;
    app.error = NULL;
    app.is_fullscreen = 1;

    //Create builder
    app.builder = gtk_builder_new();
    if( ! gtk_builder_add_from_file( app.builder, "app.glade", &(app.error)))
    {
        g_warning( "%s", app.error->message );
        g_free( app.error );
        return(1);
    }

    //Get back gtk object
    app.splashscreen = GTK_WIDGET(gtk_builder_get_object(app.builder,
                   "splashscreen"));
    app.progresslabel = GTK_LABEL(gtk_builder_get_object(app.builder,
                   "progresslabel"));
    app.progressbar = GTK_PROGRESS_BAR(gtk_builder_get_object(app.builder,
                   "progressbar"));
    app.window_main = GTK_WIDGET(gtk_builder_get_object(app.builder,
                   "window_main"));
    app.button_close = GTK_WIDGET(gtk_builder_get_object(app.builder,
                   "button_close"));
    app.button_fullscreen = GTK_WIDGET(gtk_builder_get_object(app.builder,
                   "button_fullscreen"));

    //assign a picture for each mode
    app.img_leave_fullscreen =
        gtk_image_new_from_file("icons/tools/gtk-leave-fullscreen18x18.png");

    app.img_fullscreen =
        gtk_image_new_from_file("icons/tools/gtk-fullscreen18x18.png");

    //fill the container with one picture (works find)
    gtk_container_add(GTK_CONTAINER(app.button_fullscreen),
                      app.img_leave_fullscreen);

    ...

    //Connect signals
    gtk_builder_connect_signals(app.builder, NULL); //builder

    g_signal_connect(app.window_main, "destroy", G_CALLBACK(gtk_main_quit),
                    NULL);
    g_signal_connect(app.button_close, "clicked",
                    G_CALLBACK(gtk_main_quit), NULL);

    //quit splashscreen
    g_signal_connect (app.splashscreen, "destroy", G_CALLBACK (gtk_main_quit),
                    NULL);

    //change fullscreen mode with the corresponding button
    g_signal_connect(G_OBJECT(app.button_fullscreen), "clicked",
        G_CALLBACK(change_fullscreen_mode), &app);

    g_object_unref(app.builder);

    gtk_widget_hide(app.window_main);
    gtk_window_fullscreen(GTK_WINDOW(app.splashscreen));
    gtk_widget_show(app.splashscreen);
    eventstartCommand = g_timeout_add(5, on_start_commands, &app);
    gtk_main();

    return 0;
}