This is not true for Atomic Desktops (and also not the same on KDE 
)
SDDM by default uses themes in /usr/share/sddm.
Lesson 1 in atomic desktops: /usr is immutable
The best workaround for this is here:
https://pagure.io/fedora-kde/SIG/issue/534
@siosm also mentioned the solution for this, support for multiple theme directories.
I am not sure if changing the image of a preinstalled theme would be possible when that fix is there.
It would require copying the system theme to a mutable directory and then changing it, if that works how I think it works.