I'm glad that the breeze theme allows you to specify a custom background (both on the lock screen as well as on the login screen / sddm / breeze greeter).

Unfortunately, as soon as I successfully login and the animation with the progress bar starts, it goes back to the default background.
I've looked at the sddm breeze theme, the only reference to the background I could find was in Main.qml (and the conf files) and looks pretty straight forward.
I've tried replacing default background.png in /usr/share/sddm/themes/breeze/components/artwork with my custom picture, but this still happens.

I went through the files reported by dpkg -L sddm and sddm-theme-breeze, and I couldn't find another place where the default (blurred) login background is stored, so I'm a bit confused how it can still find the default picture (unless it's cached somewhere, or baked in...).

I'm running Kubuntu 15.04 on laptop with an nvidia optimus setup, using the latest nvidia driver, but the same thing happens in a VM without any video acceleration.

run "kcmshell5 splashscreen" - i doubt they allow to set wallpapers, but you may inspect /usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/splash/*

luebking wrote:run "kcmshell5 splashscreen" - i doubt they allow to set wallpapers, but you may inspect /usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/splash/*

Thank you, there lurks indeed another copy of the blurred default wallpaper in that folder!
Replaced it, and adjusted the fillmode in Splash.qml to PreserveAspectCrop to match sddm, lock screen and the desktop - and now everything's blending together nicely.

I tried to look in to the path /usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/splash/images.
There is only a svg compressed png file, which can be opened with inkscape and it is a KDE logo. Other than that, there are no background image in it.
Users who have tried, kindly confirm if the background is customizable from there or any other method has to be adopted.

after reading ../contents/splash/Splash.qml, one can see reference to ...../contents/components/artwork/background.png

perhaps by editing this file, one can change the splash background.

Thanks for the hints!

Changing the background in
/usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/components/artwork/
did the trick for me.

Cheers!