Hello to the PP elite!

I have an image that I created. Not a photo, but a picture of my D300's focusing screen.
I have no idea on WHY I did this, other than to say I started it and then I thought tomyself I had better finish it. And now that it's finished I want to make it more flexible.

I used MS's Paint program, because 'it was there'!
(I was helping my daughter learn to draw using my old Wacom tablet, and MS Paint was easy pickn's.. so.. anyhow.....

I made a white background looking image with a black wire frame that looks remarkably (ie. exactly) the same as my D300's focusing screen.

Note! I have no uber cool image layer manipulating programs such as PS, so this has to be done via a free program of some kind. I'm guessing either or both GIMP and Paintdotnet may have the ability to do what I want(to separate the black wire frame from the white background, and save the image as a black wire frame only.

I do have FSViewer, in which I can't see any way to separate the black from the white.

I know this is about transparency in layers and so forth... but as an image manipulating dunce!.. I really have no understanding of the processes.

TIA.