We use an excellent freeware application called Screenshot by Antony Pranata, which you can install on your N-Gage-compatible device and use to take screenshots of practically any phone function, including N-Gage games.
Probably the easiest way to install Screenshot is to download it using your phone's web browser, as it will install the application directly onto the phone. Just point your phone's web browser to the following address:
http://www.antonypranata.com/screenshot/download-screenshot-symbian-os-s60
Make sure you click on the signed version, otherwise it won't work.
If you don't want to type in such a long address, just go to antonypranata.com, then click on "Screenshot", then "Download Screenshot for Symbian OS (S60)", then click on the download link for the signed version.
Using Screenshot
Screenshot works by staying running in the background while you use another application. When you want to take a screenshot, you press a particular button and the Screenshot app will create an image of whatever is on the screen at that moment.
To start Screenshot running after you've installed it, go to the phone's Applications folder and click on Screenshot. Within Screenshot select the Options menu and then Settings, this will let you adjust the behaviour of Screenshot to suit your needs.
When you're ready to take screenshots, press Back until you get back to the main menu, then run the app you want to take shots of. When you get to a suitable moment for a shot, press the button or buttons that you selected from Screenshot's Settings menu, and a message should appear when the screenshot has been taken.
When you've finished taking screenshots, hold down the phone's menu key (the squiggly swirly symbol) and select Screenshot from the list of icons. Click on Options, then Exit.
The screenshots you've taken will appear in the phone's internal memory by default, in a folder called "screenshots" within the "images" folder. They are also visible in the Gallery application if you want to browse them, and can be transferred by all the usual methods you would use to transfer photos (for example onto your computer by USB cable, onto the web by Share Online, attached to e-mails, onto other phones by Bluetooth etc).
If you want to save the shots elsewhere, you can alter where the shots are saved to from Screenshot's Settings menu.