I tried changing my login screen so it would match my desktop.. but..

Kinda didn't work.

I have an app called "Login Back" and pretty much you use it so set a back ground for the login screen if you wernt able to do so manually (as i wasn't)

But it seems I still cant change it back to even the default?

Can anyone help or guide me?

Or maybe even shed some light on the image requirements that have to be met to be used in the login screen?