PictureFillSymbol を利用した透過塗りつぶしシンボル

//PictureFillSymbol オブジェクトの作成
IPictureFillSymbol pPictureFillSymbol = new PictureFillSymbolClass();
pPictureFillSymbol.CreateFillSymbolFromFile(esriIPictureType.esriIPicturePNG, @"..\..\transparency.png");



IGraphicsContainerSelect pGraphicsContainerSelect = (IGraphicsContainerSelect)axMap;
IElement pElement = pGraphicsContainerSelect.SelectedElement(0);

IFillShapeElement pFillShapeElement = (IFillShapeElement)pElement;
pFillShapeElement.Symbol = (IFillSymbol)pPictureFillSymbol;