This is quite a quick question, if i were to have say a 4k wallpaper and I set it as my wallpaper even though my monitors max resolution is 1080p would the image look better if i resized it to 1080p or left it at its original resolution?
You will want to pick "fit to screen" otherwise you will only see about a quarter of the picture. It will not shrink to fit on your screen if you pick "center"; you will only see a 1920x1080 box out of the center of the screen. Make sense?
Looks like I missed your question entirely. It shouldn't make any difference. Just pick fit to screen and set it a your background.