i have noticed a small difference when setting wallpapers from "personalisation" and from opening a pic in explorer then right clicking and setting as desktop background.
the quality seems better if its set from explorer.
have personalisation set to fill or whatever suits the image first ,then set it from explorer,
see if it makes any difference for you
UPDATE: I found an answer here: https://superuser.com/questions/5070...w-quality-jpgs
I have not. I just did this in Windows 10 and the quality is still not improved.
I believe Windows caches its own copies of these images, but in my case it may have an expired cached copy from when I used the image with a lower resolution monitor. Setting it via Explorer did not improve the quality. Sample images attached. One of screenshot. Other of original file
from Imgur:
https://imgur.com/a/Nww0p
Original
Desktop
I think Windows re-samples images.
I have a library of wallpapers in png format.
Some of the pictures display jpg artefacts when they are used as Desktop wallpaper (in W7).
Quote