Index: app/models/attachment.rb
===================================================================
--- app/models/attachment.rb	(revision 11705)
+++ app/models/attachment.rb	(working copy)
@@ -188,7 +188,7 @@
       size = options[:size].to_i
       if size > 0
         # Limit the number of thumbnails per image
-        size = (size / 50) * 50
+        size = ((size + 49) / 50) * 50
         # Maximum thumbnail size
         size = 800 if size > 800
       else