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:
- COMPUTER CONFIGURATION > POLICIES > ADMIN TEMPLATES > CONTROL PANEL > PERSONALIZATION
- FORCE A SPECIFIC BACKGROUND AND ACCENT COLOR
Alternately you could set the following registry entry: