First off, don't use a picture box: if you do, then you are responsible for writing the text - and that includes you specifying exactly where the text should go.
You would probably be better off with a text based control such as a label or textbox.
If you must use a picturebox, then when you write the text you use the
Graphics.DrawString[
^] method, which has overloads allowing you to specify the rectange and text alignment.