Complex sentence: Chinese internet users have questioned the move of featuring Mao, instead of Chiang Kai-shek, Generalissimo of the Republic of China, in the poster, since Chiang was present at the Cairo Conference, but Mao was not.
Split into simpler sentences: Chinese internet users have questioned the move of featuring Mao's image, instead of Chiang Kai-shek, Generalissimo of the Republic of China, in the poster. They question the poster image because Chiang was present at the Cairo Conference, but Mao was not.