I recently wrote a "how to take a screenshot in Linux tutorial", and I included a shell script that helped take screenshots in GNOME with a custom application launcher on the taskbar.
While reading a book called Linux Multimedia Hacks on Safari [aff] I learned about another way to take screenshots in Linux that could replace my shell script.
You can activate GNOME's screenshot tool by invoking it from the terminal with this:
gnome-panel-screenshot
An you can even add a delay as an argument like this:
gnome-panel-screenshot --delay 10
The delay would give you enough time to open a menu or set your screen up the way you would like.
You could add this command to a custom application launcher in GNOME.

It works better than the shell script.
For more options when using gnome-panel-screenshot, type gnome-panel-screenshot --help in a terminal.
If you are using KDE, try the command ksnapshot.
See my other Linux screenshot tutorial for more options...
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Comments
RHEL/Fedora metacity golbal_keybinding
RHEL and Fedora set their keybindings to invoke the applet via PrintScreen by default.
Linux Screenshots
You can get a gnome-panel-screenshot with the PrtSc button; adding the command to the panel is only if you want a delay (for example, to open a menu that might close if you hit the PrtSc button).