alright folks i figured it out. in /usr/share/sddm/themes/breeze there is a file called theme.conf.user that had this text:
[General]
background=/usr/share/endeavouros/backgrounds/endeavouros-wallpaper.png
type=image
so i had to edit the ‘background=’ bit to add the path to the picture i actually wanted .
curiously there is another file, theme.conf that read:
[General]
showlogo=hidden
logo=/usr/share/sddm/themes/breeze/default-logo.svg
type=image
color=#1d99f3
fontSize=10
background=/usr/share/wallpapers/Next/contents/images/5120x2880.jpg
needsFullUserModel=false
i did not edit that one.