To take a snapshot of the Android screen you will need the Dalvik Debug Monitor Service (DDMS). The DDMS comes with the Android SDK. To get the SDK on either Ubuntu or Windows, check this link.
Once you obtained DDMS, locate it. It will be in your sdk tools folder:
Make sure you enable USB Debugging on the device by going to Settings -> Applications -> Development and ticking the box and plug in your device now.
select Device -> Screen Capture