Hi Lirulis,
The only way I know to do this is registering a handler for the PictureBox's Paint event, then use the GDI to draw your text on top of the picture when it is painted. Your handler will look something like this:
private void imgMap_Paint(object sender, PaintEventArgs e)
and you'll want to create the appropriate font and brush, then call e.Graphics.DrawString().
Hope this helps,
----Scott.