The website states: "You can freely use any material copyrighted by the contributors of the website, provided that:

   you limit your use to a maximum of 5% of the images or content of the website
   you quote the author
   you quote the website (as "FOTW Flags Of The World website at http://flagspot.net/flags/")
   you do not alter in any way the images or the content of the text
   you use the material for non-commercial and non-political purposes only
   if you distribute our material by a non-Internet way (e.g., floppy disks or CD-ROM) you must add this copyright text on every copy of the medium
   if you distribute our materials by a non-Internet way (e.g., floppy disks or CD-ROM) you cannot sell these media
   If you want to reuse FOTW GeoIndex maps and boundaries' data, read these rules"

The copywrite licences of Mello Luchtenberg & António Martins aren't on the internet and I have no way of knowing them, but according to the rules of the website, usage on Wikipedia should be allowed. Probably under

Це зображення не захищається авторським правом, тому що є тривіальним, не несе художньої цінності, складається тільки із загальновідомих елементів, які не мають автора.