Flag icons are made by www.IconDrawer.com
If you wish to use them on website, you have to place link to www.icondrawer.com on your site. (Somewhere on static page, not in blog or news page)
If you can not or do not want to place link back, you can purchase royalty-free license. 

ISO 3166-1-alpha-2 code elements - here
For questions buz@icondrawer.com