diff --git a/lib/redmine/wiki_formatting/macros.rb b/lib/redmine/wiki_formatting/macros.rb
index f4700f242..9ef4ffd06 100644
--- a/lib/redmine/wiki_formatting/macros.rb
+++ b/lib/redmine/wiki_formatting/macros.rb
@@ -229,7 +229,8 @@ module Redmine
         out
       end
 
-      desc "Displays a clickable thumbnail of an attached image. Examples:\n\n" +
+      desc "Displays a clickable thumbnail of an attached image.\n" +
+             "Default size is 200 pixels. Examples:\n\n" +
              "{{thumbnail(image.png)}}\n" +
              "{{thumbnail(image.png, size=300, title=Thumbnail)}} -- with custom title and size"
       macro :thumbnail do |obj, args|
@@ -239,7 +240,7 @@ module Redmine
         size = options[:size]
         raise 'Invalid size parameter' unless size.nil? || size.match(/^\d+$/)
         size = size.to_i
-        size = nil unless size > 0
+        size = 200 unless size > 0
         if obj && obj.respond_to?(:attachments) && attachment = Attachment.latest_attach(obj.attachments, filename)
           title = options[:title] || attachment.title
           thumbnail_url = url_for(:controller => 'attachments', :action => 'thumbnail', :id => attachment, :size => size, :only_path => @only_path)
diff --git a/test/unit/lib/redmine/wiki_formatting/macros_test.rb b/test/unit/lib/redmine/wiki_formatting/macros_test.rb
index 8b6f44974..70e30ef88 100644
--- a/test/unit/lib/redmine/wiki_formatting/macros_test.rb
+++ b/test/unit/lib/redmine/wiki_formatting/macros_test.rb
@@ -307,7 +307,7 @@ RAW
   end
 
   def test_macro_thumbnail
-    link = link_to('<img alt="testfile.PNG" src="/attachments/thumbnail/17" />'.html_safe,
+    link = link_to('<img alt="testfile.PNG" src="/attachments/thumbnail/17/200" />'.html_safe,
                    "/attachments/17",
                    :class => "thumbnail",
                    :title => "testfile.PNG")
@@ -316,7 +316,7 @@ RAW
   end
 
   def test_macro_thumbnail_with_full_path
-    link = link_to('<img alt="testfile.PNG" src="http://test.host/attachments/thumbnail/17" />'.html_safe,
+    link = link_to('<img alt="testfile.PNG" src="http://test.host/attachments/thumbnail/17/200" />'.html_safe,
                    "http://test.host/attachments/17",
                    :class => "thumbnail",
                    :title => "testfile.PNG")
@@ -325,16 +325,16 @@ RAW
   end
 
   def test_macro_thumbnail_with_size
-    link = link_to('<img alt="testfile.PNG" src="/attachments/thumbnail/17/200" />'.html_safe,
+    link = link_to('<img alt="testfile.PNG" src="/attachments/thumbnail/17/400" />'.html_safe,
                    "/attachments/17",
                    :class => "thumbnail",
                    :title => "testfile.PNG")
     assert_equal "<p>#{link}</p>",
-                 textilizable("{{thumbnail(testfile.png, size=200)}}", :object => Issue.find(14))
+                 textilizable("{{thumbnail(testfile.png, size=400)}}", :object => Issue.find(14))
   end
 
   def test_macro_thumbnail_with_title
-    link = link_to('<img alt="testfile.PNG" src="/attachments/thumbnail/17" />'.html_safe,
+    link = link_to('<img alt="testfile.PNG" src="/attachments/thumbnail/17/200" />'.html_safe,
                    "/attachments/17",
                    :class => "thumbnail",
                    :title => "Cool image")