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:
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...