Hi @b4xqrm7nti. Websites to find wallpapers are outside of scope of this forum (there are plenty of answers on that already on the internet).
you should keep in mind that background colors may affect your ability to disabiguate qube’s border colors. I don’t know if this is well studied, but I would encourage using either light or dark wallpapers.
A) Using the default wallpapers (easy and safe)
A lot of amazing qubes artwork already exists in dom0 in the folder (/usr/share/backgrounds/).
Just open the Settings Manager application in dom0 » Desktop
And then set the Folder: to /usr/share/backgrounds/.
B) Downloading from the internet
1. Moving image to dom0
Option a) - safest
The safest way is to open it in full screen from within a qube and then use the dom0 screenshot application to take a pic of the wallpaper. It will be saved on your pictures folder (/home/user/Pictures).
Option b) - discouraged but is “prettier”
You download the files from a qube and then move it to dom0. It’s not technically easy but you can consult it here. The security downside is that you are copying an untrusted image from the internet onto the most trusted component of Qubes (dom0)…
2. Setting background
See path A) but set the Folder to wherever in dom0 you downloaded it.