First create a Bitmap object from the file, create graphics from the image and use the DrawString method of the graphics to draw the text. Then simply use the Save method of the Bitmap object.
'First load the image
Dim b As New Bitmap("path")
'get the graphics object from the image so you can draw onto the image
Dim gr As Graphics = Graphics.FromImage(b)
'draw text
gr.DrawString("text", Me.Font, New SolidBrush(Color.Red), 10, 10)
'save the image
b.Save("path")
'dispose
gr.Dispose()
b.Dispose()