Here is one way you can do this:

Workbook wb = new Workbook();
Worksheet ws=wb.Worksheets.Add("main");
WorksheetImage wsi = new WorksheetImage(System.Drawing.Image.FromFile(MapPath("~/") + "16_excel.gif"));
wsi.TopLeftCornerCell = ws.Rows[0].Cells[0];
wsi.BottomRightCornerCell = ws.Rows[2].Cells[5];
wsi.PositioningMode = ShapePositioningMode.DontMoveOrSizeWithCells;
ws.Shapes.Add(wsi);
UltraWebGridExcelExporter1.Export(UltraWebGrid1, ws, 10, 0);

I create a workbook then a worksheet.  I then load an image then set some positioning properties.  You can also use the setBoundsInTwips method as well.  I add the image to my worksheet and then export my grid to that worksheet starting at the 10th row.