Instead OS X simply displays the default blurred image it typically uses (i.e. In this case I don’t get the background wallpaper I’ve placed into the /Library/Caches path at all. The third behavior I’ve experienced is when you suspend or hibernate the machine. This time I get again the correct wallpaper but it is somehow strangely blurred (strangely this time I get the configured two-liner to log-in). Anyway, the next screen behavior I notice is the one you get when you log out from the running system and log in again. Weirdly though OS X displays the user selection with the user picture instead of the two line login which I have selected in the system settings – but that’s another story. Here I get the background I’ve changed nice and clear in beautiful bright colors. The first one is the one you get when your machine has freshly booted. Here’s why… As far as I can see OS X Yosemite has at least three login screens-behaviours. That was helpful – but it didn’t solve the whole problem. Once the new login screen wallpaper image has been copied, close Finder, save your work in any other open OS X applications, and log out of the operating system (  > Log Out). Now find your custom renamed wallpaper image and copy it into the Caches folder, authenticating with admin credentials and agreeing to replace the existing file if requested. If so, copy and paste this file to a safe location on your Mac so that you’ll have a backup of the original if you ever want to revert to the default login screen wallpaper. Depending on your exact OS X configuration, you may or may not already have a login screen wallpaper image in this folder with the name mentioned above. This will take you to the Caches folder in the System Library.

In the box, type /Library/Caches and click Go. Next, open Finder and select Go > Go to Folder from the Menu Bar. The image must have this exact file name in order for it to serve as a custom login wallpaper. Once you’ve identified your image, save it with the filename .png.

Just open your image in Preview, go to File > Export, and choose PNG from the Format drop-down menu at the bottom of the Export window. If your image is in a format other than PNG, you can quickly convert it using the Preview app.

Os x yosemite wallpaper