Newer versions of Linux Mint use MintX as the default login screen greeter. By default it comes with a predefined collection of images. These display in the background and change periodically in a slide show type format.
The default images are great but I wanted to add a personal touch to my system and change them to some of my own photos.
Here I will run you through the process of changing MintX to use images of your choosing.
First create a directory for the new images to live in:
sudo mkdir /usr/share/backgrounds/custom/
Now using sudo, copy some images on the systems that you would like to use as backgrounds into the newly created directory i.e
sudo cp ~/Documents/Pictures/my_photo.jpg /usr/share/backgrounds/custom/
Edit the MintX slide show configuration file:
sudo nano /usr/share/mdm/html-themes/Mint-X/slideshow.conf
Scroll down past the heading [backgrounds] and delete all of the default entries. Then add in some pointing to the images you copied over i.e
file:///usr/share/backgrounds/custom/my_photo.jpg file:///usr/share/backgrounds/custom/another_photo.jpg file:///usr/share/backgrounds/custom/swesome_background.jpg
When your done save your changes, and logout. You should now see your custom images that you add being used as the background at the login prompt.