Can we please not just use the same background over and over? Maybe even tweak the theme slightly. We can go with different shades of the same color. But let's do something to mix it up.
I think the problem with that is that we have designed the whole system around a particular look. There's always been a set of colors (or at least a family of color tones) that we have used. We've had optional wallpapers before, but the default has always been something we designed. The nice thing about that is we can limit the amount of colors in it, which makes it blend better with the rest of the system. With photographs, this is much more difficult.
I think looks-wise the login screen needs a bit of an update(provided screen of 19.04 login screen). Simple changes like centering the user login and making the bar on top a darker color and 2px thinner would make the login screen a bit more complete appearance wise.