How do I just put one picture of mine as my screensaver?

Go to your pictures library, and make a new folder in your my pictures folder location,

Make a copy of the picture you want to use or drag and drop the picture you want to use in this new folder.

Right click your desktop and select personalize,

Click on the screen saver link on the bottom right,

Click on the settings listing, then click on the browse listing,

Click on the libraries listing, then my pictures, then click on the new folder you made with the file.

One way (there are bound to be others) on my W7 starter:

1. Copy the photo you wish to use to a new known folder so its on its own
2. <start orb> <control panel> <display> 
3. On the left hand side <change screen saver>
4. In the 'screen saver' choice list, pick 'Photos'.
5. Now click 'Settings' then 'Browse' and browse to the folder where you have put the photo you wish to use.
6. Click <ok> then <save> then <preview> to make sure its working OK. Finally <ok>

Hope it helps.