It depends on the version of android that you:
- For android 4.0, 4.1, 4.2, press the button "on / off" and "Down" at the same time, until the image is saved to your gallery
- For android 2.2 (froyo), press the "back" button and "on / off" at the same time
- For android 2.3 (gingerbread), press the "home" button and "on / off" at the same time
- For android 3 (honeycomb), it is your rooter machine, which depends on the exact model. Search "root model" in google.
In all cases, you can also install an application (free) to make copies as Wallpaper "No Root Screenshot It" or "Screenshot UX". In some cases (it depends of the model), it may be you need to initialize your android with an application that runs on your PC / MAC (this is explained in the manual of the android applications).
