* Open your photo in GIMP
* Go to the "Layers" menu and select "Duplicate Layer". This is like taking two copies of your photo and setting one ontop of the other.
* Go to the "Windows" menu, choose "Dockable Dialogs" and then choose "Layers".
* A new (small) window should open and you should see the two layers you now have in the "Layers" box.
* Click on the top layer in the "Layers" box (it should be called "Background copy") to make that layer the "active" one that you'll be working on, then right-click on it and a bunch of options will appear. Choose "Add Alpha Channel" (near the bottom). This will allow you to erase the top layer and see all the way through to the bottom photo in the next couple steps instead of it being white underneath.
* Go back to the window with your photo on it and click on the "Colors" menu, then choose "Desaturate". In the next window try "Average" -this usually gives a good result; it will make the top layer black and white.
* (Optional) The photo on the bottom is still in color; you just can't see it because it's under the copy we just converted to black kand white. To demonstrate that the original is still under there, click on the little eye next to the "Background" layer in the "Layers" box. That eye tells you that the layer is visible, clicking on it to remove the eye makes it invisible so you can see the layer(s) underneath. Bottom line: If you do this, you'll see the color version re-appear. When you are done, re-check the box so the top layer (the black and white one) is visible again.
* Make sure the "Background copy" layer is "active" by clicking on it in the "Layers" box.
* In the "Toolbox " window (the one with all the icons on it), select the "Eraser" (the little pink square icon).
* Erase the parts of the top layer (the black and white one) where you want to see the bottom layer (the color one) through it. (If it's a large area, you may want to do a rough selection with the lasso tool first and delete a large portion of it that way, then use the eraser tool to just do the fine details around the edges.
* Then go to the "Image" menu and choose "Flatten Image".
* Save the file and you are done!