Hey there! I figured this out...
There is a symlink called default_background.jpg in the directory:
Which links to the file:
Which is in the directory:
You are able to change the background image on the login screen by replacing the file gelsan_green.png (or changing the symlink, which is probably preferred over changing the distro files!), located in:
The image has the dimensions of:
1920 x 1200 pixels
and it's a .png. hope this helps!