The purpose of a screen saver is to make sure that an image doesn't get burned onto your screen - if you do only use one picture, it will not 'save' your screen and you will be susceptible to screen burn.
..........You could put extra copies of the same picture in the folder you are using for the the Slideshow and check the use transition effects between the shots option ............commoner.
Sorry Immi but ignore my last suggestion..........I just did one with a single picture in a folder I made called screensaver in My Documents............then do as markja says but on the settings screen move the slider to only say 60% of the screen size and use transitions.
...The pic then won't be in the same place all the time and risk screen burn..........commoner.