Screenshot

Initial Configuration
To make the Print Screen key take a screen shot: first create the following script, then copy it to and make it executable.

Install media-gfx/imagemagick.

/usr/bin/screenshot

You can, of course, modify the file path to suit your needs.

The above script can be called with three different arguments: win, scr and area. The win argument will take a screen shot of a single window. The scr argument will take a screen shot of the entire desktop, and the area argument will allow you to click and drag to capture a rectangular region of the desktop.

Please note that while the script accepts three arguments, and  behave the same. If a window is clicked, a screen shot of the window is taken. If an area is selected with the mouse by clicking and dragging, the selected region is saved as the screen shot. As such, only two arguments (scr and either win or area) are needed to achieve full functionality.

Desktop Setup
The next step is to simply tie the Print Screen key to the screen shot command in your window manager.

Xfce
In Xfce, go to the Settings Manager and then choose Keyboard. Click the Shortcuts tab, then click Add. For Command, input screenshot scr and then click OK. A second window will come up, prompting for a key combination for this command. Press and release the key. Repeat this process for the command screenshot win, but make the shortcut combination different from the previous one, e.g.,.

Openbox
Add these lines.

Save, reconfigure, and use.

KDE
Shortcuts for KDE can be easily added through the GUI.

Right-click the K menu and select "KDE Menu Editor". Once in there, custom commands can be added to the menu with the "New Item" button and have a shortcut assigned via their corresponding "Advanced" tab. All that is important is that the commands "screenshot win" and "screenshot scr" are used.

As a KDE specific alternative to this whole workflow solely

Even if this program is not yet installed it is already registered as the standard application for the print key. Thus it works out of the box.

Gnome
 

directly emerge the gnome-extra/gnome-utils

Other
The easiest way to take a screenshot of full desktop is run the follow command in your terminal. If you have media-gfx/imagemagick you can also run:

XMonad
Bind keys for "/usr/bin/screenshot scr" and "/usr/bin/screenshot win" in xmonad.hs. One possibility for this is as follows:

Save, then recompile and reload xmonad (by default, meta-q to recompile and reload in one operation), and test it out.