You can try this;
void makePDF()
{
Response.ContentType = "application/pdf";
Response.AddHeader("content-disposition", "attachment;filename=test.pdf");
Response.Cache.SetCacheability(HttpCacheability.NoCache);
string imageFilePath = Server.MapPath(".") + "/images/test.jpg";
iTextSharp.text.Image jpg = iTextSharp.text.Image.GetInstance(imageFilePath);
// Page site and margin left, right, top, bottom is defined
Document pdfDoc = new Document(PageSize.A4, 10f, 10f, 10f, 0f);
//Resize image depend upon your need
//For give the size to image
jpg.ScaleToFit(3000, 770);
//If you want to choose image as background then,
jpg.Alignment = iTextSharp.text.Image.UNDERLYING;
//If you want to give absolute/specified fix position to image.
jpg.SetAbsolutePosition(7, 69);
PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
pdfDoc.Open();
pdfDoc.NewPage();
Paragraph paragraph = new Paragraph("this is the testing text for demonstrate the image is in background \n\n\n this is the testing text for demonstrate the image is in background");
pdfDoc.Add(jpg);
pdfDoc.Add(paragraph);
pdfDoc.Close();
Response.Write(pdfDoc);
Response.End();
}