* Open your photo in Paint.NET
* 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.
* Now you should see the two layers in the "Layers" box in the lower right hand corner of your Paint.NET workspace. Click on the top layer in the "Layers" box to make that layer the "active" one that you'll be working on.
* Go to the "Adjustments" menu and select "Black and White". This makes 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 on the bottom. To demonstrate this, uncheck the box next to the top layer in the "Layers" box. Removing the checkmark makes that layer invisible so that you can see the layer(s) underneath. 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) reappears.
* Make sure the top layer is "active" by clicking on it in the "Layers" box.
* In the "Tool" list, select the "Eraser"
* 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.
* When you are finished, for safety, you may want to save it as a .pdn file -at least temporarily.
* Then go to the "Layers" menu and choose "Merge Layer Down" -this pastes the two copies of your photos together so it can be saved as one image. BUT BE CAREFUL!!! This step cannot be undone! (This is why saving it as a .pdn file first is a good idea.)
* Save the file as a .jpg or whatever to upload to the web (.jpg files don't support layers, so you have to merge it down first).