If you have the image on your server - and if it's accessible from C# code it is - then it's simple:
using (Image im = Bitmap.FromFile(Path.Combine(imagesPath, "MyPic.jpg")))
{
using (Graphics g = Graphics.FromImage(im))
{
g.DrawString("HELLO!", myFont, myBrush, xLocation, yLocation);
im.Save(Path.Combine(imagesPath, "MyPicWrittenOn.jpg", ImageFormat.Jpeg));
}
}