There is nothing more fantastic than get familiar with the terminal and linux commands. The way to get it it is very, very easy. Remember, you have to be root or use your SuperCow powers!
Go to usr/share/backgrounds as root. In my case I click right button with the mouse in the /linuxmint folder and open it in terminal.
Now, you will use the following command line:
$sudo ln -s [TARGET WALLPAPER] default_background.jpg
This will create a symbolic link to the background you want as your Lock Screen and Login. As an example:
$sudo ln -s /home/~your_local/Images/blue.jpg default_background.jpg
This should wok with all linux distros.
Merry Xmas everybody!
This is a special post, since this problem is bothering a lot of people. So I'll make it in English.
The lock screen (Gnome based) on Ubuntu Linux cannot be easily changed. There is no GUI for it and no easy way with GConf. How I manage to do it, simple way:
First, choose the image you want to be your background on the Lock Screen. Copy the file.
Now go to usr/share/backgrounds as root.
You should see some other folders in here. As I am using Linux Mint, my folder is /linuxmint. Open the folder. In here you should see a couple of files, probably image files.
One of these image is signaled as a link. Just leave it alone. The other should have a very suggestive name: default_background.jpg
Paste your chosen image into the folder. Now come the tricky part! :P
Rename the older image with the name you want or just delete it. Generally, the image has a copy on other folder. If you don't want to loose it, just copy to another location.
Now, get the image you want as your new background and just rename it to default_background
If you did right, pressing CTRL + ALT + L, your screen will probably Lock with the new background! YEY!
Any doubts, just comment.