How do some programs change the windows background and screensaver automatically from time to time?

How do that program change the background and screensaver. If it stores the path of the files in the windows folder please specify the place where it stores the path of the files in the window folder.



I want to make a program in C that can do the same job. But for that I want to know where windows saves the file.How do some programs change the windows background and screensaver automatically from time to time?
It would modify some registry keys. You can look at the registry using regedit.exe.

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