How can I kill or refresh Desktop Wallpaper with a Terminal command?
You could try restarting dock which, amongst many things, draws the wallpaper on the desktop. But be careful; as dock also controls a lot of window management apps may 'un-minimise' or un-hide and windows. As dock also controls Mission Control, you won't be able to change spaces or go into Mission Control until dock restarts again (which is mostly instantly).

From terminal, try:

killall Dock