If it's not the admin account, the wallpaper after a restart is probably associated with the root account. By default the root account is disabled. Be very careful with the root account because you will have free rein to really screw up the machine. Follow these Apple instructions to enable your root account, then log into that account and change the wallpaper, and finally follow Apple's instructions to disable the root account.
for a single account, used to be several free third party tools can set the image, as well as remove/restore blur.
Tinkertool used to do this, at least up to OS 10.11. Looks like that feature got pulled, likely due to OS changes. I see this instruction, that probably works for 10.12 and 10.13. Have not tested it.
It used to fairly easy, but seems to be more complex these days. Admin discussion and a couple work-arounds here.