The default greeter for Xfce is lightdm. Currently, the login window's position is fixed and can't be changed without hacking the code. I've put in a feature request at https://bugs.launchpad.net/bugs/1147914 to make this adjustable so that those of us who use a custom background can also place the window where we want it. It just occurred to me that the more people commenting that they'd like this, the greater the probability that the devs will do something about it. If you'd like to have Yet Another Way to customize your computer, add a comment to the bug, even if it's only Me Too.
Registered Linux user #470359
Permanently recovered BOFH
Any advice in this post is worth exactly what you paid for it.