If the ugly image appears, it seems to be because no other image is chosen, so the system defaults to the ugly dynamic ones. What I did is below. I know it seems to conflate the login with the lock screen, but it works. My lock screen image is the same as my desktop image. That might help. Just in case, you can have a different lock screen image if you want. That process is below.
Changing the Login Image
- Open a new FINDER window.
- Choose the STARTUP disc under LOCATIONS on the left. (It is whatever you called it during setup.)
- Click the main volume in the window. (Again, this is whatever you named it.)
- Choose USERS, pick the user folder (whatever you named it), and select the PICTURES folder.
- Paste the image desired for the login screen to this PICTURES folder.
- Close all windows.
- Right click DESKTOP and choose CHANGE DESKTOP BACKGROUND.
- Under FOLDERS on the left choose PICTURES and click the file just copied.
Note: If you do not want a photo, you can just choose a color from the COLOR. Either way, it seems important to actively click something so that there is a blue selection frame around it. Choosing the CUSTOM COLOR option will give you a DESKTOP BACKGROUND, but it does not seem to affect the LOGIN SCREEN.
9. Test it by logging out.
Changing the Lock Screen Image
This process is documented elsewhere in the thread, but...
- Open a new FINDER window.
- Choose the STARTUP disc name under LOCATIONS on the left. (It is whatever you called it during setup.)
- Choose main volume in the window. (Again, this is whatever you named it.)
- Choose the LIBRARY FOLDER and then CACHES, DESKTOP PICTURES, and then the folder with the long alphanumeric name.
- Paste the photo for the lock screen in there. Make sure it is named "lockscreen.png."
Note: Remove all other images from the folder except for lockscreen.png. Otherwise, nothing will change. Also, sometimes, it may require a restart to complete wipe any of the cached information.