In development mode, if an avatar is missing just serve up a placeholder

config/environments/development.rb

@@ -43,7 +43,9 @@ Discourse::Application.configure do
   config.enable_mini_profiler = true
 
   require 'middleware/turbo_dev'
+  require 'middleware/missing_avatars'
   config.middleware.insert 0, Middleware::TurboDev
+  config.middleware.insert 1, Middleware::MissingAvatars
 
   config.enable_anon_caching = false
 end

lib/middleware/missing_avatars.rb

@@ -0,0 +1,26 @@
+module Middleware
+
+  # In development mode, it is common to use a database from a production site for testing
+  # with their data. Unfortunately, you can end up with dozens of missing avatar requests
+  # due to the files not being present locally. This middleware, only enabled in development
+  # mode, will replace those with an appropriate image.
+  class MissingAvatars
+    def initialize(app, settings={})
+      @app = app
+    end
+
+    def call(env)
+      if (env['REQUEST_PATH'] =~ /^\/uploads\/default\/avatars/)
+        path = "#{Rails.root}/public#{env['REQUEST_PATH']}"
+        unless File.exist?(path) 
+          default_image = "#{Rails.root}/public/images/d-logo-sketch-small.png"
+          return [ 200, { 'Content-Type' => 'image/png' }, [ File.read(default_image)] ]
+        end
+      end
+      
+      status, headers, response = @app.call(env)
+      [status, headers, response]
+    end
+  end
+
+end