Drawing a grid on a picture

[Jive Dadson]

Hello folks.  I know precisely zero about image processing.  I want to 
draw a grid of lines one or two pixels wide on a picture (.jpg, .png, or 
whatever). [I want to transfer a sketch of the picture to a canvas (for 
oil painting), using the "grid method."]

I figure this is probably a very easy thing to do, but I can't seem to 
find the documentation I need.  It all seems to assume you already know 
something.  I have wxPython installed.  I also have the wxPython demo. 
The section "Using Images/Image" looked promising.  It just shows how to 
  read a picture and display it as a bitmap.

Can someone show me how, or point me in the right direction?

Thanks a lot.