Windows saves the list of wallpapers ever used on the system in a registry Key.
HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersion
ExplorerWallpaperMRU
All those Registry Binary values here are the address to the images you used as wallpapers.

The last part in this binary makes no sense to me but I think this i information about the dimensions and style of the image.
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