Well, I use my own custom theme, never used the breeze one.
I tested it it and was able to change the background image by clicking on the background (Hintergrund) symbol (plasma systemsettings5).
If this dosen't work , you should be able to change the background by editing /usr/share/sddm/themes/breeze/theme.conf
Be careful, make a copy of the original file like cp theme.conf theme.conf~
and you need to know the path to the picture or the right colour code.