This is a simple change to make, once you know how.

Right-click on an empty part of the Desktop and then left-click on Properties. Left-click on the Desktop tab.

The backgrounds for the Desktop supplied with Windows are listed in the bottom half of the window. To add your own, left-click on the Browse button. Find the picture you want to use, left-click on it once and then click on Open.

If it is a photo it is probably too big to all fit on the screen at once. There are some options in the Position menu that can help. Click on the down arrow and select stretch, as this will squeeze the picture to fit.