Instructions
Open the "My Pictures" folder, and select the picture you want to use as your screen saver. Write the file name down on a piece of paper so that you select the correct picture later.
Click the "Start" button in Window's 7, and select "Control Panel" and "Appearance and Personalization" from the drop-down menu.
Click "Personalization" and then "Screen Saver" from the new windows that open up after each click. Select "Photos."
Click "Settings," and select the photo you wrote down. Click "Save" and "OK" to make the picture the screen saver on your computer.