To take a screenshot from the screen of a Android device, a commandline tool /system/bin/screenshot is present. At least on my device, a (Aldi) AT-AS43D, running Android 4.0.3. So, to get a screen shot on your PC from Android, you can use the following code:

adb shell screenshot /sdcard/myscreenshot.png
adb pull /sdcard/myscreenshot.png myscreenshot.png
adb shell rm /sdcard/myscreenshot.png

On my device however, the screenshot image looks wierd. Apparently the red and blue channels in the image are swapped by the screenshot tool. So I found a easy way to swap the colors back, using 'convert', which is part of the imagemagick package (on Ubuntu at least). So the code becomes something like this:

adb shell screenshot /sdcard/myscreenshot$$.png
adb pull /sdcard/myscreenshot$$.png /tmp/myscreenshot$$.png
adb shell rm /sdcard/myscreenshot$$.png
convert /tmp/myscreenshot$$.png -separate -swap 0,2 -combine myscreenshot.png
rm /tmp/myscreenshot$$.png

This all has been tested on a Ubuntu PC. Other environments, especially Micro$oft Windows, may need adaptation. Installation requirements: adb (google for it, or install Android SDK); ImageMagick.

Alternatively, you can apparently press 'volume-down' and 'power' to get a snapshot, since Android 4.0
And, a cheap Android 2.3 device does not have the 'screenshot' tool. The tool, copied from my working device does in principle work, but most data is not shown.
Apparently, on some 2.3 devices, a 'snapshot' option is in the menu that comes up when holding the 'power' button. Alas, not on my cheap one.
Also apparently, on some 2.2 devices, pressing the 'power' button while holding the 'home' button takes a screen shot. But not my 2.3 one.