How to add currency flags in WordPress?

Sometimes I got messages from the customers where described that currency flags in WordPress doesn work. There is one thing they forget to do while adding flags into settings of currencies - selection of flag file URL instead file attachment, look on the screens please:

Just click once on 'File URL' button and that is all :)

Also read this: https://wordpress.stackexchange.com/questions/9727/link-to-file-url-by-default