First, right-click on the desktop and select Personalize.
In the Personalize window that appears, click on Desktop Background located in the lower left corner of the window.
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Then, in the Desktop Background window, hover your mouse over the image you want to add as the background image, click on the tick in the upper left corner of each image.
You can also click the Select All button to select all images. When you select two images, Slide Show will automatically turn on and Change picture every will be displayed. You click on the list below the text Change picture every and select a time period displayed between the two images (here I choose 15 minutes to display a picture). You can click on the shuffle box to randomly replace the picture. When you are done, click Apply changes to finish.
If Windows 7 product key does not move the image according to the fade effect, you must right click on the Computer icon on the desktop, select Properties. Click on Advanced Systems Settings. In the System Properties window that appears, go to the Advanced tab and click the Settings button in the Perfomance pane. In the Performance window, select the Animate controls and elements inside windows option. Press OK twice to confirm.
Now my desktop will automatically transfer the background image after 15 minutes, the transition effect is very nice and smooth.