I want to explain how you can take a screenshot with a Sony Xperia Smart Phone. I need some screenshots sometimes and had the same question. It took me some time to find out how I can save screenshots. So let me explain:
You must push the “Volume Down” and “Standby Key (Lock Key)” at the same Time.
Important: I experienced that this really needs to be done in the same moment. There will not be a screenshot saved, if you hit one of the keys faster than the other.
You can take a screenshot on Android with the Sony Xperia Smart Phone nevermind what kind of app or window you have opened. It will work everywhere. I hope this helped you to understand how to use this nifty function of your Sony Xperia. It is sometimes necessary to take a screenshot and it is done fast if you push the two mentioned keys.
You will find your screenshot in your photo gallery and indeed in the photo folder of the Sony Xperia memory. If you have connected your Sony Xperia to your computer then you could copy the screenshot also to your computer hard drive, like you do it with your photos.
Hope that helped.
EDIT: I noticed that you now just need to push the power off button since android jelly bean. In the menu you can then choose “take a screenshot”.