Before telling you where to click, I would highly suggest you save the images you want to using as a desktop wall paper (or slide show) into the Windows Desktop folder, or a subfolder. This will make it easy to locate them and ensure they aren't accidentally moved. Then...
- Click on the Start button
- Click 'Control Panel'
- Type 'desktop background' in the search box and click 'Change desktop background'.
- Select the image you want to use; if it isn't in the list of photos, find an image in the 'Picture Location' list for other categories, OR
- Click 'Browse' to search your computer for the photo you want to use as a background and double-click it.
- You can also adjust the way the image is displayed on your screen; you can use stretch, fit, tile or center. To do so, under 'Picture position', click the arrow and click on the option you wish to use.
- Don't forget to save your changes by clicking 'Save changes'.