Create a screensaver

Assuming you are using windows XP on this:
Create a new folder in "my pictures " folder
Copy the images you want to use into there
On your desktop, RIGHT click and choose "properties" to open the display settings
Click "Screen saver" and choose my pictures slideshow
Click "settings" to open the options for this
Click browse to point to where you saved the images
Set the duration and timings to suit your tastes
Exit by clicking Ok and the OK to exit the display settings

The photos in your computer can be used as screen savers (those files that pop up when you are idle).
1. Right click on your desktop and choose personalize.
2. Select Screen Saver
3. Select Photos
4. Click OK

All of the photos in your pictures folder will be your screen saver.

If what you want is this picture to be your background (the file that shows on your desktop all of the time)
1. Right click on your desktop
2. Select personalize
3. Select Desktop Background
4. Change the "Location" to Pictures from its setting of Windows Backgrounds
5. Select a picture
6.Click OK. It is automatically sized to your desktop.