The filename of the flags should be "File:Flag of (country).png". In some images, there would also be .svg before .png. In a case where there are two flags of the same country that has .svg and .png and .png only, just pick .png only. Also, don't forget to add "the" before the country in the filename if the country has "the" on its name (e.g. the Philippines, the Netherlands, the United States).