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

public static extern int DwmUpdateThumbnailProperties(IntPtr hThumbnail, ref DWM_THUMBNAIL_PROPERTIES props);

Public Declare Function DwmUpdateThumbnailProperties Lib "dwmapi.dll" (ByVal hThumbnail as IntPtr, ByRef props as DWM_THUMBNAIL_PROPERTIES) As Integer

Used with DwmUpdateThumbnailProperties.