To us more experienced Linux Users ...
Wallpapers, however non-important we may deem them to be, Windows Émigrés would usually consider it part of the "switching the team loyalty" process, to seek out and
-
choose an Linux-/Ubuntu-related wallpaper, or
-
choose a completely new wallpaper, unrelated to anything they previously had,
in which case, they would like the Distro to make it easier for them to access those ... directly ... for selection from within their newly-adopted environment.
To the newbies, which we truly welcome "into the fold" ...
There are many sources. I do however strongly recommend that you consider the wallpaper used for the
- Desktop
to be different from the other two wallpapers that can be set for each of
- Login Screen, and
- Screensaver.
The one on the Desktop, in my view should be of a mostly uniform colour, so that the icons that are placed on the Desktop do stand out clearly from the background, to avoid the "search and find" issue on a very busy wallpaper background.
Again only my opinion, if you leave your computer on when you walk away, your Login Screen should be your "voice", putting your "best foot forward", how you want others to perceive you! Think ...
- serious?
- dedicated?
- whimsical?
- creative?
- playful?
- loyalties?
- congenial?
- part of team?
- specialization?
My own wallpaper for Login Screen is shared with the community in the following discussion:
My Desktop is the exact same blackish background ... but with no UbuntuMATE wordmark. 
Lastly, if you choose to enable a Screensaver, the image(s) associated with that can again become a vehicle to show off your personality, preferences and tastes. Above all, those should be "in good taste", because you never know who may "sneak up on you" while your computer has gone idle!
For those who are fortunate to have multiple displays, but whose displays may not be of identical size/shape, the following
will offer you an attractive option to generate the likes of this:
For viewing actual wallpapers to choose from, some sites to offer examples of search possibilities to find others:
- Wallpaper Competition for Impish Indri - Ubuntu 21.10 - Desktop - Ubuntu Community Hub
- https://ubuntu.com/blog/tag/wallpaper
- wallpapers ubuntu - Google Search
... and for those who are curious as to how Ubuntu/Canonical became what they are, you are invited to visit this following link to start that "Journey of Discovery":