added argument to end of image request to change default if nonexistent

srctweb/_plugins/gravatar.rb

-# from https://tlattimore.com/blog/using-gravatar-in-jekyll/
+# modified from https://tlattimore.com/blog/using-gravatar-in-jekyll/
 require 'digest/md5'
 
 module Jekyll
   module GravatarFilter
     # Add our new liquid filter.
     def get_gravatar(input)
-      "//www.gravatar.com/avatar/#{hash(input)}"
+      "//www.gravatar.com/avatar/#{hash(input)}?d=mm"
     end
 
     private :hash
@@ -18,4 +18,4 @@ module Jekyll
   end
 end
 
-Liquid::Template.register_filter(Jekyll::GravatarFilter)
\ No newline at end of file
+Liquid::Template.register_filter(Jekyll::GravatarFilter)