uploads: add limits on max image dimensions.

config/danbooru_default_config.rb

@@ -179,6 +179,21 @@ module Danbooru
       35.megabytes
     end
 
+    # Maximum resolution (width * height) of an upload. Default: 441 megapixels (21000x21000 pixels).
+    def max_image_resolution
+      21000 * 21000
+    end
+
+    # Maximum width of an upload.
+    def max_image_width
+      40000
+    end
+
+    # Maximum height of an upload.
+    def max_image_height
+      40000
+    end
+
     def member_comment_time_threshold
       1.week.ago
     end