Changing the desktop wallpaper also changed the login screen. Follow these steps to override that behavior and reset the login screen back to the default.
cd /usr/share/glib-2.0/schemas/
sudo vi com.canonical.unity-greeter.gschema.xml
Look for this key node:
<key name="draw-user-backgrounds" type="b">
<default>true</default>
<summary>Whether to draw user backgrounds</summary>
</key>
Change the value true to false.
You can also remove the white dots by changing this key node:
<key name="draw-grid" type="b">
<default>true</default>
<summary>Whether to draw an overlay grid</summary>
</key>
Now recompile the schema with this command:
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
Log out and back in to see the change.