On Android, you first need to define an emulator if you don’t have one. You do this by launching Eclipse ADT > Window > Android Virtual Device Manager.
One you have defined your emulator, you can check your Cordova app on it by:
cordova build android
cordova emulate android
It takes ages for the emulator to launch with your app so be patient…
After your app loads and you want to take a screen shot from the emulator, go back to the eclipse window and from the menu:
Window > Show View > Other. In the newly opened dialog box, under Android category, select Devices.
Now on the Devices panel, Click the camera button – you’ll figure it out from there.
On iOS things are simpler – run the app in the emulator by opening the project in XCode, selecting the type of device emulator and running your app. Command-S will save a screen shot on your Mac desktop.