Was excited to read this, since Yosemite killed our custom login background for all our clients with the blurred mess. No luck for me on this though; the login screen is the blurred El Cap background and the lock screen is gray :-(

@TomDay - really? Mine are showing up crystal clear. Not sure why I'm seeing this and you're not...
I'll take a pic and post it up.

EDIT: Just did on a freshly updated Mac.

Wow yea that looks great. I'll try deleting a couple of .plists.
-Tom

FYI, I tried it on a 10.11 release build and 10.11.1 seed build. Both working on two different MBP's...

Got the login screen background working now! The file had "no access" for
Everyone, needs 'read". Now just have to get the lock screen background to
match now, its still gray. I'll play with it this week @ JNUC. Thx for the help.

@scottb Really happy with the way this is working! getting ready to release 10.11 to our community and wanted to get a custom background image out to Everyone when they update. Are you doing this? Any idea how to get one deployed?

@TomDay - you could easily use Composer for this I think.
Get your image, name it, give the right permissions, then open Composer. Should be built with the read perm's as you found and then save as a package since it's not a /User file, but a system file.

@scottb Thx, yes Composer definitely worked for this! Had to create a blank pkg and manually build the directory structure though, then add the file. Attached a screen shot of what were are rolling out. Simple but Everyone seems to like it!

Hey @TomDay , were you or anyone able to figure out changing the background when the screensaver kicks in on El Capitan? It is still blurred grey for me.