Here's how I'd approach it. Pull your pic into GIMP and resize/crop it the way you want. Print that picture on photo paper. Back in GIMP, hide your picture layer (click they eyeball). Then on a new layer, fill that with a light gray. Run your puzzle filter on the gray layer. Print your gray layer.
To cut your puzzle.
Mount your photo on BB Ply with your favorite mounting technique.
Cut out the gray background pattern and mount that to some thick cardstock or a cereal box cardboard. Trim to size.
Cut another piece of thick cardstock or cardboard to size
Sandwich your project blank cardboard (bottom), photo/BB Ply (middle), cardboard with puzzle template (top)
Wrap with clear packing tape to hold it all together.
Cut.
Now your puzzle won't have any of those lines, in case you drift off a bit.