With the Ice Cream Sandwich (ICS) update for the Galaxy S2 a few functions changed which included taking a screenshot with the Galaxy S2. Previously and before the ICS update, a user had to hold down the home button and the power button simultaneously. It was a case of hit or miss and more times than often it was miss.
Taking a screenshot with the Galaxy S2 is easy.
Press the volume button and the power button simultaneously and the screenshot will be saved as a JPEG to your screenshots folder in gallery.