Using GIMP. Open the image.
Color / Desaturate
Filters / Edge-Detect / Edge…
I just used whatever was standard settings = Sobel, 2.0, Smear.
Color / Invert
Color / Curves
Put a dot near the middle and then drag the right side up.
Lets put a border on it.
-Select / shrink, 3 pixels
-Select / invert selection
-Pencil Tool, choose black, draw over the edge
Change print size. Image / print size / 8″ wide (or 10.5″ tall).
And print your coloring book page.