The lock screen background of mate on ubuntu mate 15.04 is specified as ‘[org.mate.background] picture-filename’ in /usr/share/glib-2.0/schemas/ubuntu-mate.gschema.override’. To override ubuntu-mate.gschema.override, you should create a gschema override file whose filename comes after ‘ubuntu-mate’ in alphabetical order.
[org.mate.background] is defined in ‘/usr/share/glib-2.0/schemas/org.mate.background.gschema.xml’.
Simply, create ‘/usr/share/glib-2.0/schemas/z-mate.gschema.override’. Since ‘z’ in ‘z-mate’ comes after ‘u’ of ‘ubuntu-mate’ in alphabetical order, ‘z-mate’ will be applied after ‘ubuntu-mate’, so ‘z-mate’ will override ‘ubuntu-mate’.
The content of z-mate.gschema.override should be
[org.mate.background]
picture-filename=’/path/to/background’
Now, you have to compile gsettings schemas with the command below.
`sudo glib-compile-schemas /usr/share/glib-2.0/schemas/’
After compiling schemas, restart your display manager or your system. The lock screen background will have changed.