diff --git a/prosopopee/prosopopee.py b/prosopopee/prosopopee.py
index a3bec37..360b0e7 100644
--- a/prosopopee/prosopopee.py
+++ b/prosopopee/prosopopee.py
@@ -167,7 +167,7 @@ class Image(object):
         return ""
 
     def generate_thumbnail(self, gm_geometry):
-        thumbnail_name = ".".join(self.name.split(".")[:-1]) + "-" + gm_geometry + self.name.split(".")[-1]
+        thumbnail_name = ".".join(self.name.split(".")[:-1]) + "-" + gm_geometry + "." + self.name.split(".")[-1]
 
         source, target = os.path.join(self.base_dir, self.name), os.path.join(self.target_dir, thumbnail_name)