Problem to display flag icon

Hello realloc,

If you have some time, I would be thankfull if you could help me with a small problem i am experiencing in your plugin. How can i change the flag icon to show the Greek flag on this site? It seems that the American flag must be the default because on the Multisite Language Switcher Options the |Custom URL for flag-images| is blank. I tried unsuccessfully to edit the url so it shows the greek flag. Please take a look.
http://alevrasp.gr/en/
Username and pass: test

Last but not least, your work is amazing and much appreciated.
Thanks in advance.

https://wordpress.org/plugins/multisite-language-switcher/