Engine中IPictureElement背景如何设置为透明？

IPoint pt = new PointClass(); 

pt.X = mapx; 

pt.Y = mapy; 



IPictureMarkerSymbol pngPictureMarkerSymbolCls = new PictureMarkerSymbolClass(); 

pngPictureMarkerSymbolCls.CreateMarkerSymbolFromFile(esriIPictureType.esriIPicturePNG, @E:picture.png); 

pngPictureMarkerSymbolCls.Size = 50; 



IScreenDisplay screenDisplay = actview.ScreenDisplay; 

screenDisplay.StartDrawing(screenDisplay.hDC, (System.Int16)ESRI.ArcGIS.Display.esriScreenCache.esriNoScreenCache);



screenDisplay.SetSymbol(pngPictureMarkerSymbolCls as ISymbol); 

screenDisplay.DrawPoint(pt); 

screenDisplay.FinishDrawing();