Change Loginscreen in 10.12

@Lars - This still works for me on 10.12.2 beta 5, not sure why it would be broken for you. had the same background from 10.9 - 10.12.2 b5

http://sachinparmarblog.com/changing-the-login-screen-wallpaper-on-mac-os-x/

Ok, this is interesting. Was this a clean install or a update from 10.x to 10.12? I´ve made a new image installation with a 10.12 vanilla image built with autodmg. And then the loginpic was installed with a package. Same way like in 10.10 or 10.11.

I have been using the same method for changing the login screen background for the last 3 or 4 versions of the OS

I have created a package (dmg) that replaces the com.apple.desktop.admin.png file in the /Library/Caches/ folder

The package is deployed after reboot (install on boot drive after imaging)

Hope that helps

I have tried it serveral times, but for me it is not working in 10.12.1 (16B2659). I have prepared a clean installed system and i tried it with the package (replaces the com.apple.desktop.admin.png file in the /Library/Caches/ folder) that worked fine in 10.10/10.11. And i tried it directly to drop a com.apple.desktop.admin.png file, too. Only way to get the loginpic changed is to drop a sierra.jpg in the Folder Library/Deskop Pictures. But this Loginpic is then horrible blured. If i delete my sierra.jpg and only drop my com.apple.desktop.admin.png inside the Library/Cache folder i only get a black login screen.

I second what @stevevalle said. You can't install the login wallpaper (dmg) during imaging. It has to install during the first reboot to work correctly.

@ssrussel That is the way i do that. After imaging, with a firstinstall policy that get active a few minutes after the first systemstart. Worked fine in 10.9, 10.10 and 10.11.

Don't know if this will be helpful for the OP or not but I found out through some trial and error that if the PNG doesn't have an Alpha Channel it might not work.

If you Get Info on the PNG and it says under "More Info" that "Alpha Channel: No", then it likely won't work. Not 100% sure that's the only factor, but I was using Pixelmator to make a PNG and it wasn't working when I exported the PNG to be used as a login window wallpaper. When I export from Photoshop CC it includes the Alpha Channel and it works. I also make my PNG files 5120x3200 resolution (but I doubt that matters).

Hopefully that'll save someone a headache in the future.

@Lars have you looked at this?

@ssrussel In my PNG there is no Alpha Channel and it worked with 10.10, 10.11. And if you go to the post from Sachin_Parmar and open the link to his blog. There is his working png file to download. There´s no Alpa Channel inside, too. I think it must be anything different in 10.12, If i install by loginpic package i get a black screen. I think this could be the missing alpha channel. But in 10.10, 10.11 it was not needed to work fine.

@bentoms yes, you find the webadress in brackets in my opening question. It has not worked for me, the loginpic was always blurred. I get the pic only changed if i edit the Library/desktop pictures/ sierra.jpg. I do not understand why some people have no problem to put a com.apple.desktop.admin.png in the cache folder in 10.12. As you see in your link, it seems a little bit difficult.

@Lars Yes, I didn't have any problems applying a custom login window wallpaper in 10.10 and 10.11 with any PNG I used in the /Library/Caches location named com.apple.desktop.admin.png with the proper permissions, etc, with or without an Alpha Channel.

I was having issues applying one in 10.12 so I had to figure out why it wasn't working. It could be something else, but the only thing I noticed the difference between a working 10.12 loginwindow wallpaper and a non-working one was that Alpha Channel.

Here are is a PSD (Photoshop File) I used to create my Login Window Wallpaper. I created a blurred background, but put my company logo on the wallpaper non-blurred:
https://dl.dropboxusercontent.com/u/8949569/Example_Blurred_LoginWindow.psd

Here is a png that doesn't have an Alpha Channel (won't work in 10.12 as loginwindow wallpaper)
https://dl.dropboxusercontent.com/u/8949569/Example_Blurred_LoginWindow_NoAlpha.png

Here is a png that does have the Alpha Channel (works in 10.12 as loginwindow wallpaper)
https://dl.dropboxusercontent.com/u/8949569/Example_Blurred_LoginWindow_YesAlpha.png

In Photoshop, if I go to "File > Save" and choose PNG as the format it'll include the Alpha Channel. If I go to "File > Export for Web" it won't apply the Alpha Channel.

If you were to take [this]((https://dl.dropboxusercontent.com/u/8949569/Example_Blurred_LoginWindow_YesAlpha.png) PNG and rename it to com.apple.desktop.admin.png, put it /Library/Caches and change the permissions to sudo chown root:wheel /Library/Caches/com.apple.desktop.admin.png and sudo chmod 644 /Library/Caches/com.apple.desktop.admin.png log out and it should work.

@ssrussell Thats it! Now it works and without a blurred Logo. Thank you very much for your detailed information.

thanks @ssrussell You've solved our issue. The alpha channel fixed it.

I am still having problems getting our custom login screen to work in 10.12.5. I've done everything suggested here, but nothing seems to be working. I would like to get this resolved before the start of the school year.

@Damon.Byg When are you applying the altered com.apple.desktop.admin.png file? During Netbooted imaging? Do you DEP image? Policy? I found that if I install the altered login window wallpaper during the actual NetBoot session it never gets applied. I actually had to add my .dmg or .pkg of the com.apple.desktop.admin.png file during the first reboot, with a low priority, like 15 or 20. See if this helps in your workflow.

It's a policy that runs once a day on all of our student machines. I flushed the policy for the test machine multiple times and it copies the file successfully, but it won't change the login screen.

Is your policy installing the png inside a PKG or DMG? If it is a PKG, I'd suggest having a postinstall script that'll fix the permissions on the file. If it is a DMG, I'd add a script to the policy or use the "Files and Process" payload and run the "Execute Command" section to do something like:

sudo chown root:wheel /Library/Caches/com.apple.desktop.admin.png
sudo chmod 644 /Library/Caches/com.apple.desktop.admin.png

I already have a script setup to run those exact commands after the file gets copied to the computer. Everything says it works, but when the test machine reboots it still comes up with the default Sierra login screen. When I go look at the /Library/Caches/com.apple.desktop.admin.png file it's my custom login screen.

@Damon.Byg If you are comfortable with it, would you be willing to email me your wallpaper or posting a link to it here so I can examine it to make sure it's not the png itself? My email is steven_russell [at] beaverton [.k12.or.us]

@ssrussell Do you mind if I email you about a similar issue that I believe is related to permissions?

Sure @uncommonsense