In most cases the flags were applied as decals supplied from the USA , they could be stuck on when a pilot had his photo taken either for himself or for propagander
purposes. A few senior commanders like McCampbell kept them on for a while. They were also used on warships for planes shot down eg. on gun turrets.
See attached piece.
Nick
http://www.fahnenversand.de/fotw/flags/xf-vict.html