Well, according to Microsoft you cannot currently change the background image on Windows 8.1 Login Screen. I confirmed this with Microsoft Partner Support HERE . I believe this will change in future versions of Windows.
That being said you can force a specific colour (or color for our American friends!) by using Group Policy:
Alternately you could set the following registry entry:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Accent
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To quickly change the background pic of the Win 8.1 login screen you'll need to access the (C:\ProgramData\Microsoft\Windows\SystemData\S-1-5-18\ReadOnly\LockScreen_Z) folder and replace the JPEG inside with the picture you want using the exact same filename. Because the SystemData is protected by the OS, is protected so you'll need to be an admin and you will have to take ownership of that folder and also give the local SYSTEM account full access to it with propagation. These changes all get reset when you logout to see if the new picture shows up like its supposed to.