Customize The OS X Yosemite Login Screen
Give the OS X Yosemite login window a personalized look using your own image. This is a simple trick to do. Just save your desired image as:
Then, just log out to apply the change. You should immediately see your new image. If you don’t make sure you put it in the correct Library folder and that the permissions are correct.
This trick works best with wallpapers that have the image left or right-justified so that the login fields are not obstructed. Brighter colors also help since the login window dims them a bit.
You can use Photoshop or the likes to make the image blurred to give it that pure Yosemite feel. But some images work better than others. The Portal image below looks cool and is blurred, but the login fields are a little difficult to distinguish.
If you want to use an image with alpha channels (i.e. transparency) such as one used to change the Apple Logo on the Mavericks login window, it won’t work as well. For some reason, the transparent areas show up as red.
I’m not a Photoshop pro, but you can use this template (1920×1080) which has a layer with the opacity reduced so you can line up where the login fields will go. You could use this to create your own wallpaper, or simply to line up where the login fields and buttons will appear on your wallpaper. Your results may vary depending on the resolution of your Mac.