There was an error running gnome-screenshot:
Failed to execute child process "gnome-screenshot" (No such file or directory)
I'm running a fresh CentOS 6.3 system installed using the x86_64 Live CD which shortly after installing it noticed that it didn't came with Disk Usage Analyzer. Putting the error message and the missing application points the problem to the package that provides both gnome-utils.
To fix the problem and also install the other missing applications:
# yum install gnome-utils -y
And that's it!