Login Screen Background Switcher - Checks for and creates backups

I was looking for a way to change the files as easily as possible while preserving the original apple image and not messing with any reference files. I had initially scripted it as an event that could be called from a selected file in the finder window, which would still be nice, though that would require checks for multiple file selections etc. This seems to work more efficiently.

The code was pieced together from bits and pieces of other code on this site but not stolen outright as I have made major changes to the way the event is handled. The end result is a script that asks the user to select a file from a dialog box (jpeg only for compatibility) and copies it to the folder /System/Library/CoreServices/ renaming it to DefaultDesktop.jpg.

There are many excessive additions I put in the dialog box message to help me debug as it went but I left them in to make it a solid script with notices given for each change made. I am not sure if there is any changes needed to squeeze the code down but it is fully functional and as far as can be seen, error free (the most important part).

One addition that would be nice is to have the prompt for the admin password to be active.

Any advice is welcome.
This is my first attempt at using AppleScript.

set newImagePath to quoted form of POSIX path of (choose file with prompt "Choose a JPEG image for the login window background/" of type {"public.jpeg"} without invisibles)

set imagePath to "/System/Library/CoreServices/DefaultDesktop.jpg"
set imagePath_org to "/System/Library/CoreServices/DefaultDesktop_org.jpg"

tell application "Finder"
	if exists imagePath_org as POSIX file then
		set msg to "Backup already exists, "
		if exists imagePath as POSIX file then
			set msg to msg & "Original Exists!"
			set msg to msg & "however, no original was found. Your image has taken its place."
		end if
		set msg to "Backup wasn't found, "
		if exists imagePath as POSIX file then
			set msg to msg & "however, an original exists and was used to create a backup!"
			do shell script "mv " & imagePath & " " & imagePath_org with administrator privileges
			set msg to msg & "and no original exists either. Your image will be used as the backup the next time this script is run."
		end if
	end if
	do shell script "cp " & newImagePath & " " & imagePath with administrator privileges
	display dialog msg & "
The login screen background has been changed, log out to see results!" buttons {"OK"} default button 1
end tell

Model: MacBook Pro
Operating System: Mac OS X (10.6)