Disable screen blur on fast user switching in Yosemite
Yes, you can change the blurred background image on the login screen in Yosemite.

In root /Library/Caches (make sure you're not in the user ~/Library) you will find a file called: com.apple.desktop.admin.png. This is the blurred image that was created when you set your Desktop wallpaper.

Replace this file with any image you would like on your login page. Make sure to keep the permissions the same as the original file.

Hat Tip to Bobby T from Apple Support Communities: https://discussions.apple.com/message/26879262#26879262

Cheryl