There is no tool to easily select a background image to be displayed as your login (GDM – Gnome Desktop Manager) screen. There are, however, a number of ways to change the background with the following method probably being the easiest for novice users.
As root, place the image that you wish to use as your login background in /usr/share/backgrounds/images or somewhere similar. Ensure that your background image is appropriately sized for the screen that you are using. You can check your screen geometry as follows:
$ xdpyinfo | grep -B1 dot
Then invoke gnome-control-center as user gdm as follows:
$ sudo su - # xhost +SI:localuser:gdm # sudo -u gdm gnome-control-center
In gnome-control-center, click on the background option and select the image that you want displayed as your backgound when the login screen is displayed.
When you have selected the desired background image, exit gnome-control-center, logout and you should see the background that you selected.