Find & Save a Picture
Find a picture you want to use on the internet. If you do a Google search, look for "wallpapers", they tend to be bigger.
To have a picture as your desktop background, you want to find one that is 1920 x 1080 (= the size of our monitors). Anything smaller may be blurry.
Click on the thumbnail and then right-click on the preview/full-size image to "Save image as..."
Save to your Pictures folder.
Set Desktop Background
Option #1: picture big enough
Go to your Pictures folder...
Right-click on the picture and choose “Set Desktop Background”.
Option #2: picture too small
Right-click anywhere on your desktop and go to "Personalize".
Click on Desktop Background at the bottom...
Browse to your Pictures folder...
Select the picture(s) you want to use...
You can change Picture Position so the picture is not stretched but centered, and you can also have a bunch of pictures that change automatically for you. When you're done, Save.
Note: Do not change your theme to one of the "high contrast" ones!