change key used to generate chapter image file names

MediaBrowser.Model/Entities/DisplayPreferences.cs

@@ -1,14 +1,20 @@
 using MediaBrowser.Model.Drawing;
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 
 namespace MediaBrowser.Model.Entities
 {
     /// <summary>
     /// Defines the display preferences for any item that supports them (usually Folders)
     /// </summary>
-    public class DisplayPreferences
+    public class DisplayPreferences : INotifyPropertyChanged
     {
+        /// <summary>
+        /// Occurs when [property changed].
+        /// </summary>
+        public event PropertyChangedEventHandler PropertyChanged;
+        
         /// <summary>
         /// The image scale
         /// </summary>