When taking a screenshot, there is a gray image displayed.
Before upgrading to Ubuntu 13.04 the screenshot was working properly, however, after the upgrading process the screenshot wont work no more. I tried to verify which package was the problem from: gnome-screenshot, I unistalled it and installed back on my laptop, but it wont work anyways.
you can also try the handy ...
I got the same problem, even with Shutter, the only way I found to be able to fix it is to reset Unity.
Open a terminal and run the following commands:
Easy to do, but not suitable for everyone I guess.
Source: http://www.omgubuntu.co.uk/2013/04/how-to-reset-unity-compiz-in-ubuntu-12-10-and-13-04
My problem was that gnome-screenshot was never installed..
So I ran
and obviously it solved my problem.