For instance, you can always take a screenshot of your desktop any given time by simply pressing the "print-screen" button on your Keyboard. But say that you were accessing a certain menu in your Totem video player or the main programs menu in Gnome, then try pushing the "print-screen" button at the same time ... nothing happens :(.
Although as an answer, as mentioned before, you can use a dedicated application for that but if all you want is, "just" to take a screenshot (while accessing a menu) then you don't have to install any other software, just follow the below method and you should be able to get that "full" screenshot with ease!.
*. From the main menu, go to: "Applications" > "Accessories" > "Take Screenshot"
Now from the window that you get (as shown below), choose your preference (either to grab the whole screen, current window only or define the area manually) but under the "Grab after a delay of" option, set the seconds that the screenshot taking app's waiting time (5-10... or any seconds you like).
You can also choose things like "include pointer" and "include widow border" and add few effects as well.
So as a general rule, it's a wise thing to set it above 5 seconds so you can navigate in your menu. After that press "Take Screenshot" button, go to your menu location and simply wait.
Once the app automatically takes your screenshot after the delay is complete, it'll open itself in a new window in which you can give a location where to save, etc. That's it. Hope this helped :D.