Taking screenshots in the Gnome desktop environment is as simple as pressing [Print Screen] key. If like me you use a Mac Mini USB keyboard (which doesn’t have such a key) you could mess around with keyboard mapping.

Alternatively within Gnome there is a screenshot app called “gnome-screenshot” found in Application > Accessories > Take Screenshot. I’ve placed a copy of this app on the lower panel (the camera icon) and assigned a 10 second delay to the launch properties. This gives me time to prepare for taking screenshots, especially useful for taking a shot of the 3D cube within Beryl.
Gnome screenshot launch properties

If you use the XGL/Beryl environment there is a neat screenshot plugin which allows you to take a screenshot of a specific area through the combination of pressing the [Super] key & left clicking your mouse whilst high-lighting the area. You may need to edit the plugin properties within the Beryl Setting Manager to set your desired directory path such as /home/username/Desktop.

Beryl Settings Manager screenshot plugin properties

Here is an example of the selected area (minus mouse cursor) I choose to take a screenshot of within the Beryl environment.

Beryl screenshot selection area

After releasing the [Super] key and mouse button the screenshot of the area you selected should now be automatically saved in the directory you assigned.

Resulting screenshot