Thanks for contacting support.
You may load your image from stream, save it temporarily to some physical location, add it as a background of cell and delete after saving the PDF documents. You may achieve this by something like following:
System.Drawing.Image img = new Bitmap(imgStream); // imgStream is your image stream
img.Save(dataDir + "SavedImage.jpg");
Header.Cells.BackgroundImageFile = dataDir + "SavedImage.jpg";
// Delete the saved file after document is saved.
document.Save(dataDir + "OutputTable.pdf");
File.Delete(dataDir + "SavedImage.jpg");
In case there is more of your requirement, please feel free to let us know.