Replace local image references with placeholders for deployment

src/exercises/__init__.py

@@ -796,7 +796,7 @@ def show_exercise_4():
                 """)
                 
                 # Diagramme interactif
-                st.image("Gemini_Generated_Image_fk7im5fk7im5fk7i.png", 
+                st.image("https://via.placeholder.com/400x300/4A90E2/FFFFFF?text=Robot+Souple+Schema",
                         caption="Schéma du robot souple - Rapport étudiant")
         
         with col2:

src/introduction/__init__.py

@@ -35,7 +35,7 @@ def show_introduction():
         """)
     
     with col2:
-        st.image("Gemini_Generated_Image_fk7im5fk7im5fk7i.png",
+        st.image("https://via.placeholder.com/400x300/4A90E2/FFFFFF?text=Digital+Twin+Architecture",
                 caption="Architecture d'un jumeau numérique")
     
     # Démonstration interactive simple