diff --git a/prosopopee/prosopopee.py b/prosopopee/prosopopee.py
index 1520ce6..daf103d 100644
--- a/prosopopee/prosopopee.py
+++ b/prosopopee/prosopopee.py
@@ -91,7 +91,7 @@ class Video(object):
         return ""
 
     def generate_thumbnail(self, gm_geometry):
-        thumbnail_name = ".".join(self.name.split(".")[:-1] + ["-", gm_geometry, ".png"])
+        thumbnail_name = ".".join(self.name.split(".")[:-1] + ["-%s" % gm_geometry, "png"])
 
         source, target = os.path.join(self.base_dir, self.name), os.path.join(self.target_dir, thumbnail_name)
 
@@ -165,7 +165,7 @@ 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] + ["-%s" % gm_geometry, self.name.split(".")[-1]])
 
         source, target = os.path.join(self.base_dir, self.name), os.path.join(self.target_dir, thumbnail_name)