diff --git a/include/gdiplusflat.h b/include/gdiplusflat.h
index 4041c2c..3cc6d26 100644
--- a/include/gdiplusflat.h
+++ b/include/gdiplusflat.h
@@ -328,6 +328,7 @@
 GpStatus WINGDIPAPI GdipGetImageItemData(GpImage*,ImageItemData*);
 GpStatus WINGDIPAPI GdipGetImagePixelFormat(GpImage*,PixelFormat*);
 GpStatus WINGDIPAPI GdipGetImageRawFormat(GpImage*,GUID*);
+GpStatus WINGDIPAPI GdipGetImageThumbnail(GpImage*,UINT,UINT,GpImage**,GetThumbnailImageAbort,VOID*);
 GpStatus WINGDIPAPI GdipGetImageType(GpImage*,ImageType*);
 GpStatus WINGDIPAPI GdipGetImageVerticalResolution(GpImage*,REAL*);
 GpStatus WINGDIPAPI GdipGetImageWidth(GpImage*,UINT*);