That was my question, and it happened again. So I did what you said, only now I've encountered a problem. It seems to have erased all of the default backgrounds and kept the pictures I have under "My pictures" instead.
On the left, expand HKEY_LOCAL_MACHINE, then SOFTWARE, then Microsoft, then Windows. Select CurrentVersion.
On the top menu, go to edit -> new -> string. Name the new value "WallPaperDir", without quotes, caps sensitive.
After naming the value to "WallPaperDir", double click it and put "%SystemRoot%WebWallpaper", without quotes, in the "value data" field
Then download this file, unzip it, and put the 100 images somewhere in your "my pictures" folder: [Link](Mouse over link to see full location)
They can be in a subfolder, but they need to be unzipped. I'm not sure if you can mark the folder as hidden.
As long as you keep the 100 images in your "My pictures" folder, the problem should be fixed.
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