The easy way of setting the default background is, Use System Preferences to set your background to what you want the default to be.
You need to keep the background image in a shared location, /Library/Desktop Pictures/ works fine for this.

then open the terminal and issue these commands to copy and set the proper permissions:
[code]sudo cp ~/Library/Preferences/com.apple.desktop.plist /System/Library/User\ Template/English.lproj/Library/Preferences
sudo chmod 600 /System/Library/User\ Template/English.lproj/Library/Preferences/com.apple.desktop.plist
sudo chown root:wheel /System/Library/User\ Template/English.lproj/Library/Preferences/com.apple.desktop.plist[/code]

For the finder again, set the prefs how you what them and issue these commands.
[code]sudo cp ~/Library/Preferences/com.apple.finder.plist /System/Library/User\ Template/English.lproj/Library/Preferences
sudo chmod 600 /System/Library/User\ Template/English.lproj/Library/Preferences/com.apple.finder.plist
sudo chown root:wheel /System/Library/User\ Template/English.lproj/Library/Preferences/com.apple.finder.plist[/code]

It is best to use defaults, or Property List Editor.app and double check the keys in these plists before you copy them.
Some may have paths, others may have “remembered data” like last items used, or last server connected to.