This is too difficult to start guessing. To me this game is about finding out which city it is without having to guess randomly, but by taking small clues from the pictures like flag, specific buildings, surrounding nature, company logo's, etc. This means that a skyline might be guessed quickly, but when not even one player can guess it (almost) correct within two try's the skyline is too difficult. This is a rule I use for myself.
Personally I loose interest in this game since there seems to be trend of finding as difficult as possible and to be honest, if that's the purpose I can find a skyline within minutes that won't be guessed correctly for the next few months. To me the challenge in this game is finding a skyline that's both difficult to guess, but still has so many clues that within a few hours there have been a dozen guesses.