Replace an original image by a drawn image (C#)

Question

Hi! I open an image in a picture Box: pictureBox.Image=original_image; and then I change it using some transformations and show it: e.Graphics.DrawImage(..). My question is: how do I replace the original image by the one I draw in the pictureBox? I guess the real question is how to create a Bitmap with only the drawn image and then make: pictureBox.Image=resized_image. Thanks!