These scripts make changes to your loginwindow preference file. If the file doesn't exist it will be created.
This preference is set for the computer (all users) as it effects the login window we all see when we log in. The preferences are stored in a plist file /Library/Preferences/com.apple.loginwindow.plist
You could use terminal commands to add these things yourself - the scripts just add a confirmation dialog box and make it easier for novice users.
For the picture script the image needs to be a jpeg and needs to be stored in a public folder as the system needs access to the file before a user logs in.
The picture will be squished or stretched to fit on the screen.
I've made these mini apps in Automator and I've included the workflow files in case you want to edit them.
Once the automator workflow checks it is OK to run the the users is asked for input, the results of this are passed to some shell scripts.
for the loginwindow text
/usr/bin/defaults write /Library/Preferences/com.apple.loginwindow LoginwindowText "New loginwindow text"
and for the desktop picture
/usr/bin/defaults write /Library/Preferences/com.apple.loginwindow DesktopPicture "PathToFile/pictute.jpg"