There's a lot of options, simple one, just stick a image in
/usr/share/desktop-base/active-theme
or
/usr/share/sddm/themes/breeze
then use systemsettings > startup & shutdown > login screen > background > load from file, point to your image
I don't bother with theme.conf, because when sddm updates, you loose your settings,(image, face, etc) at least that's what happens to me