External monitor wallpapers are back

For the longest time, the wallpapers on my external monitors just didn't show. Where there should have been a wallpaper, there was solid black.

Nothing else was wrong with the laptop, so when a few restarts didn't fix it, I didn't bother looking into it further. (It's an aesthetics issue and I usually have windows over it anyway, so who cares.)

I finally looked into it this morning, and you just have to restart the dock:

killall Dock

Some supervisor process somewhere restarts the dock and boom, my wallpapers are back. I'm not sure why a restart didn't do the same thing. Weird.