This submission [Pause Wallpaper SlideShow] has been marked a duplicate of
A lot of times it's happened to me that I'm watching the wallpaper and I want it to stay (pause the slideshow), because I like it or whatever other reason I got.
Right now all you can do is create a new set of Wallpapers with just that single wallpaper. If you want the slideshow back, you switch to the other Desktop Wallpaper set thus starting over with the slideshow, not to mention the hassle of having to do all those steps.
Add a "Pause SlideShow" or a "Freeze Desktop Background" option (I like the last better) right below "Next Desktop Background" in the Desktop's right-click menu.