static extern int DwmRegisterThumbnail(IntPtr dest, IntPtr src, out IntPtr thumb);

Declare Function DwmRegisterThumbnail Lib "dwmapi.dll" (ByVal Dest as IntPtr, ByVal Src as IntPtr, ByRef Thumb as IntPtr) As Integer

Thumb = Handle to the dwmthumbnail. This variable will be populated by DwmRegisterThumbnail, and is to be used with DwmUpdateThumbnailProperties and DwmUnregisterThumbnail.

Thumb = Handle to a dwmthumbnail provided by DwmRegisterThumbnail.

hThumbnail is a handle to a thumbnail created by DwmRegisterThumbnail.