Wallpaper image gets duplicated

I have an iPhone 3G. I have selected one of the wallpaper images supplied in the phone as my wallpaper. When I sync, this image gets into Photo Roll. Next time there are two there, and so on. But there is nothing in my iPhoto on my computer (Mac mini, OS 10.8.5). Why is tghis happening, and how can I prevent it?

Thank you in anticipation of help.