Linux Mint comes with default Gnome and MATE desktop environment. To install an additional K desktop environment (KDE):
sudo apt-get install kde-full
This is a huge package which is 1.4GB, this is the easiest way to install KDE if you have enough HDD spaces.
You can remove the kdm if you want, for me I preserve kdm but chose lightdm (Linux Mint default dm) as default.
To change lightdm background edit the file:
sudo gedit /etc/lightdm/unity-greeter.conf
Look for this line:
background=/usr/share/backgrounds/linuxmint/default_background.jpg
Change the absolute path to the path your preferred image (jpg or png) is located.
cyruslab 
Thanks for the how-to! Im still very new to linux and im getting the hang of sudo code. Its good to know that I was close to the code by trying to install with help, all i needed was “-full” 🙂
Thanks. I am glad this short post helps. Next time if you want to find a package using debian based linux use apt-cache search command.
I have been wondering how to install groups of applications forever in Linux Mint. Any help there? The kde-full helps, but I’d like to know if the “full” works for any group of apps.
Thanks.
kde-full is the meta package which includes those K apps and groups, I am not sure about your question, are you saying if the suffix “full” applies for other packages you install? If this is what you mean then I would say use apt-cache search to find out the name of the package.