Wow I'm impressed both with the quiz and with getting 6 out of 7 :D but I am no that sure about the one I missed, Finland, can you really say it's hidden within the Norway flag?
I've edited the instructions to make it clearer as your or Kolji didn't understand the premise. Look at the link in my comment above to see what I mean.
kirbykart, ConnardLeBarbant and muradgoat mean that if you remove the blue cross you get the Danish flag, but you can't do this just by cropping so it is impossible.