public CIMFill ConstructPictureFill( 
   string fileName,
   double size
)

/// <summary>
/// Constructs a picture fill with the specified parameters.
/// ![ConstructPictureFill](http://Esri.github.io/arcgis-pro-sdk/images/Symbology/ConstructPictureFill.png)
/// </summary>
/// <returns></returns>
private static Task<CIMPolygonSymbol> CreatePictureFillPolygonAsync()
{
    return QueuedTask.Run<CIMPolygonSymbol>(() =>
    {
        CIMStroke outline = SymbolFactory.Instance.ConstructStroke(CIMColor.CreateRGBColor(110, 110, 110), 2.0, SimpleLineStyle.Solid);
        //picture
        var pictureFill = SymbolFactory.Instance.ConstructPictureFill(@"C:\CaliforniaEmblem.png", 64);

        List<CIMSymbolLayer> symbolLayers = new List<CIMSymbolLayer>();
        symbolLayers.Add(outline);
        symbolLayers.Add(pictureFill);
        return new CIMPolygonSymbol() { SymbolLayers = symbolLayers.ToArray() };
    });
}