Рабочий код для этого
System.Drawing.Image originalImage = dpgraphic.image;// replace your image here i.e image bitmap
//Create empty bitmap image of original size
float width=0, height=0;
Bitmap tempBmp = new Bitmap((int)width, (int)height);
Graphics g = Graphics.FromImage(tempBmp);
//draw the original image on tempBmp
g.DrawImage(originalImage, 0, 0, width, height);
//dispose originalImage and Graphics so the file is now free
g.Dispose();
originalImage.Dispose();
using (MemoryStream ms = new MemoryStream())
{
// Convert Image to byte[]
tempBmp.Save(ms, System.Drawing.Imaging.ImageFormat.Jpeg);
//dpgraphic.image.Save(ms, System.Drawing.Imaging.ImageFormat.Bmp);
byte[] imageBytes = ms.ToArray();
}