Basically, you don't: you write onto the image that the picture box displays.
Handle the PictureBox.Click event and you get the X and Y position from the EventArgs that the handler is provided with.
Then get the image, and draw your text onto it:
using (Graphics g = Graphics.FromImage( MyPictureBox.Image))
graphicImage.DrawString(theText, theFont, theBrush, theLocation);