Olive Sorceress
After you save an image to your pictures folder, the quick and easy way to set it as wall paper is to right click on the opened image and select "set as background" or a similar phrase.
You can also go to Personalize in setup or right click on the desktop to select Personalize. In Windows 10, it is still fritzing because it is in beta testing. Updates are still fixing the Personalize problems.
Mike1942f
You put the image you want on your computer and then go to control panel (in Windows) and Display to tell windows where to find the image.
For your desktop, you can use any images you find on the Internet - I would use Google Images 'flowers' to find thousands of them.