diff --git a/app/models/post.rb b/app/models/post.rb
index deb29b043..7bb198d09 100644
--- a/app/models/post.rb
+++ b/app/models/post.rb
@@ -1402,7 +1402,7 @@ class Post < ActiveRecord::Base
       ModAction.log("undeleted post ##{id}")
     end
 
-    def replace!(url)
+    def replace!(url, replacer = CurrentUser.user)
       # TODO for posts with notes we need to rescale the notes if the dimensions change.
       if notes.size > 0
         raise NotImplementedError.new("Replacing images with notes not yet supported.")
@@ -1436,11 +1436,8 @@ class Post < ActiveRecord::Base
         self.source = upload.source
         self.tag_string = upload.tag_string
 
-        ModAction.log(<<-EOS.strip_heredoc)
-          replaced post ##{id}: #{image_width_was}x#{image_height_was} (#{file_size_was.to_formatted_s(:human_size)} #{file_ext_was.upcase}) -> #{image_width}x#{image_height} (#{file_size.to_formatted_s(:human_size)} #{file_ext.upcase})
-          source: #{source_was} -> #{source}
-          md5: "#{md5_was}":[/data/#{md5_was}.#{file_ext_was}] -> "#{md5}":[/data/#{md5}.#{file_ext}]
-        EOS
+        comments.create!({creator: User.system, body: presenter.comment_replacement_message(replacer), do_not_bump_post: true}, without_protection: true)
+        ModAction.log(presenter.modaction_replacement_message)
 
         save!
       end
diff --git a/app/presenters/post_presenter.rb b/app/presenters/post_presenter.rb
index eaee5e70e..69d74bb33 100644
--- a/app/presenters/post_presenter.rb
+++ b/app/presenters/post_presenter.rb
@@ -279,4 +279,53 @@ class PostPresenter < Presenter
     pool_html << "</li>"
     pool_html
   end
+
+  def comment_replacement_message(replacer = CurrentUser.user)
+    "@#{replacer.name} replaced this post with a new image:\n\n#{replacement_message}"
+  end
+
+  def modaction_replacement_message
+    "replaced post ##{@post.id}:\n\n#{replacement_message}"
+  end
+
+  def replacement_message
+    linked_source = linked_source(@post.source)
+    linked_source_was = linked_source(@post.source_was)
+
+    <<-EOS.strip_heredoc
+      [table]
+        [tbody]
+          [tr]
+            [th]Old[/th]
+            [td]#{linked_source_was}[/td]
+            [td]#{@post.md5_was}[/td]
+            [td]#{@post.file_ext_was}[/td]
+            [td]#{@post.image_width_was} x #{@post.image_height_was}[/td]
+            [td]#{@post.file_size_was.to_s(:human_size, precision: 4)}[/td]
+          [/tr]
+          [tr]
+            [th]New[/th]
+            [td]#{linked_source}[/td]
+            [td]#{@post.md5}[/td]
+            [td]#{@post.file_ext}[/td]
+            [td]#{@post.image_width} x #{@post.image_height}[/td]
+            [td]#{@post.file_size.to_s(:human_size, precision: 4)}[/td]
+          [/tr]
+        [/tbody]
+      [/table]
+    EOS
+  end
+
+protected
+
+  def linked_source(source)
+    # truncate long sources in the middle: "www.pixiv.net...lust_id=23264933"
+    truncated_source = source.gsub(%r{\Ahttps?://}, "").truncate(64, omission: "...#{source.last(32)}")
+
+    if source =~ %r{\Ahttps?://}i
+      %("#{truncated_source}":[#{source}])
+    else
+      truncated_source
+    end
+  end
 end
diff --git a/test/unit/post_test.rb b/test/unit/post_test.rb
index 68f45764a..13939b714 100644
--- a/test/unit/post_test.rb
+++ b/test/unit/post_test.rb
@@ -1593,6 +1593,9 @@ class PostTest < ActiveSupport::TestCase
       Delayed::Worker.delay_jobs = true                    # don't delete the old images right away
       Danbooru.config.stubs(:use_s3_proxy?).returns(false) # don't fail on post ids < 10000
 
+      @system = FactoryGirl.create(:user, created_at: 2.weeks.ago)
+      Danbooru.config.stubs(:system_user).returns(@system)
+
       @uploader = FactoryGirl.create(:user, created_at: 2.weeks.ago, can_upload_free: true)
       @replacer = FactoryGirl.create(:user, created_at: 2.weeks.ago, can_approve_posts: true)
       CurrentUser.user = @replacer
@@ -1638,6 +1641,14 @@ class PostTest < ActiveSupport::TestCase
       should "log a mod action" do
         assert_match(/replaced post ##{@post.id}/, @mod_action.description)
       end
+
+      should "leave a system comment" do
+        comment = @post.comments.last
+
+        assert_not_nil(comment)
+        assert_equal(User.system.id, comment.creator_id)
+        assert_match(/@#{@replacer.name} replaced this post/, comment.body)
+      end
     end
 
     context "replacing a post with a pixiv html source" do