If you'll need to have a screen captured on your T-Mobile Samsung Galaxy S2 (S II) SGH-T989, that's too very easy. You don't need to have it rooted or install any software tools or screenshot capture application just to have a screenshots of your phone.
You can directly take a screenshots right away from your device because the Samsung Galaxy S2 is already has it's own built in screen capture function. All you need to do is just - by pressing the HOME button and then quickly press and release the Power button. (you have to push the power button quickly because the task manager comes up if you keep the home button pushed too long)
This will also works on AT&T Samsung Galaxy S II SGH-I777 and Samsung Galaxy S II I9100 (global version). This is how tech bloggers and developers do when they want to share a screenshot sample of their newly develop android themes or custom ROMs out over on the internet.