I needed to run my Selenium tests on a virtual machine, on RedHat Enterprise Linux 6.6 in a multi-screen environment and this messed up the screenshots that the tests took. Virtual frame buffer to the rescue:
First I had to install it,
sudo yum localinstall xorg-x11-server-Xvfb-1.10.4-6.el6.x86_64.rpm
then I could start it,
Xvfb :1 -screen 5 1024x768x8 &
make that terminal session use it,
and I could finally run my tests, with the screenshots working fine.