In Plasma 5.9 the theme.conf of the breeze SDDM theme contains these settings:

[General]
type=color
color=#1d99f3
background=

When customizing the background image with SDDM KCM there is a new file "theme.conf.user" in the SDDM theme directory:

[General]
background=foo.png

However, when restarting SDDM the custom background picture is not shown.

I think the theme.conf.user file has to look like this:

[General]
type=image
background=foo.png

Then it works normally.