Support loading of device images from paths instead of keeping them in memory

src/calibre/devices/interface.py

@@ -380,7 +380,9 @@ class BookList(list):
       3. size (file size of the book)
       4. datetime (a UTC time tuple)
       5. path (path on the device to the book)
-      6. thumbnail (can be None)
+      6. thumbnail (can be None) thumbnail is either a str/bytes object with the
+         image data or it should have an attribute image_path that stores an
+         absolute (platform native) path to the image
       7. tags (a list of strings, can be empty).
     '''
 

src/calibre/gui2/library.py

@@ -1418,9 +1418,12 @@ class DeviceBooksModel(BooksModel):
         data = {}
         item = self.db[self.map[current.row()]]
         cdata = item.thumbnail
-        if cdata:
+        if cdata is not None:
             img = QImage()
-            img.loadFromData(cdata)
+            if hasattr(cdata, 'image_path'):
+                img.load(cdata.image_path)
+            else:
+                img.loadFromData(cdata)
             if img.isNull():
                 img = self.default_image
             data['cover'] = img