Update: If you Galaxy S II is already upgraded to Ice Cream Sandwich Android 4.0.3 ( or other version of 4.0), to do screen shot: Press at the same time the Volume Down button and Power button for about 2-3 seconds.
What we mean here by using Home and Power keys combination on making screenshot in Samsung Galaxy S II GT I9100 is that there is a built in screenshot application feature on this SGS2 without additional trick or applications needed. Just using the combination of Home key and Power Key, you would able to capture the screen display on your Galaxy S2 and automatically stored and saved onto SGS2’s gallery images. Although there is also available for software to do screenshot on the Android Market (Google Play).
Here are the tutorial and instructional steps that you had to follow:
Note to success: Make it the 4 steps being done less than A SECOND