How does Win7 automatically switch desktop wallpapers?

Dressing up the Win7 desktop, it makes it a bit new, and then attracts the mother's interest in the pc. I told my mother that it would make the desktop look like a face, automatically "change", she was curious, how does it seem like a face change? Now I began to teach my mother desktop wallpaper automatically converted.
Now, let's talk about how this wallpaper automatically changes.
First, right click on the desktop and select “Personalization”.

Select Personalization
and click on the following "Desktop Background" to enter.

Select desktop background (slide show)
Select some wallpapers for slideshows.

Check the picture
Finally set the replacement picture interval, let it change to the next picture.

Setting the picture interval
Mom doesn't know what is going on with this change of face, I will explain it in a slap in the face, in fact, it is like holding a stack of pictures, one by one Look, as long as you set the time, it will automatically replace it. In the end, the mother realized that it was the same thing.
Now the mother is happy to find some of the photos taken before, set them up, and automatically convert them on the desktop.