You cannot directly change something in the GUI thread from a different thread. The reason you are getting a segfault is probably because the GUI thread and your "Thread" are not synchronized.

The correct way to do it is to emit a signal from your Thread that is connected to a slot in MainWindow.

Qt Code: 
  1. class MainWindow : public QMainWindow
  2. {
  3. // ...
  4. private slots:
  5.   void onChangeLabel( int mode );
  6.  
  7. private:
  8.   QLabel * m_label;
  9. };
  10.  
  11. class Thread : public QThread
  12. {
  13. // ...
  14. signals:
  15.   void changeLabel(int mode );
  16. };
  17.  
  18. MainWindow::MainWindow( QWidget * parent )
  19.     : Qwidget( parent )
  20. {
  21. // ...
  22.     connect( m_mainwindowThread, &Thread::changeLabel, this, &MainWindow::onChangeLabel );
  23. }
  24.  
  25. void Thread::afficheLabelArret()
  26. {
  27.     if (m_threadGainable.labelModeFroid == true) {
  28. 	emit changeLabel( 0 ); 
  29.     else if (m_threadGainable.labelModeChauffage == true) {
  30. 	emit changeLabel( 1 );
  31.     } else {   
  32.         emit changeLabel( 2 ); 
  33. }
  34.  
  35. void MainWindow::onChangeLabel( int mode )
  36. {
  37.     switch( mode )
  38.     {
  39.         case 0:
  40.             m_label ->setPixmap(QPixmap("/home/ludo/Qt/test2/build/images/froid.jpg")); 
  41.             break;
  42.  
  43.         case 1:
  44.             m_label ->setPixmap(QPixmap("/home/ludo/Qt/test2/build/images/chauffage.jpg"));
  45.             break;
  46.  
  47.         default:
  48.             m_label ->setPixmap(QPixmap("/home/ludo/Qt/test2/build/images/abigael.jpg"));
  49.             break;
  50.      }
  51. }
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The signal / slot mechanism will make sure that the inter-thread call is synchronized correctly.