For some reason, in order to switch my wallpaper, I need to reboot my pc every time, and if I am not doing that, it will just mark the wallpaper I chose as "selected", but won't actually change until I reboot.
Is there any solution to that?
The same problem was solved for Loki previously.
You have to restart (or at least reload) Gala to change the wallpaper. To do that, you can type or paste the following into Terminal:
killall -HUP gala
-HUP flag tells the command to basically "Hang-UP" the gala process, which makes it restart.