MX18 How to change greeter background image

Anyone know how to change the MX18 greeter background image. Its so low contrast I cant see the login properly.
Which greeter program does Mx18 use ? lightdm, xfce ?

To determine which display manager you're using on a Debian-family distro:

cat /etc/X11/default-display-manager

we use lightdm.

the image is defined in /etc/lightdm/lightdm-gtk-greeter.conf

Thanks both, it helped me solve the problem.