Add logo for Twitter.

assets/images/polyguin.sh

@@ -187,6 +187,12 @@ chown 1000:1000 icons/gen_polyguin_color_favicon.ico;
 magick convert -background none -gravity center -resize 96x96 -extent 96x96 polyguin_color.svg gen_polyguin_color_avatar.png;
 chown 1000:1000 gen_polyguin_color_avatar.png;
 
+##
+# Twitter avatar is 400x400 square. MUST be square.
+#
+magick convert -background none -gravity center -resize 400x400 -extent 400x400 polyguin_color.svg gen_polyguin_color_twitter.png;
+chown 1000:1000 gen_polyguin_color_twitter.png;
+
 ##
 # WordPress theme screenshot is 1200x900. MUST be this.
 #