Creating a jigsaw puzzle with The GIMP

Set up the image

Open the image.

Add a new layer, with a white background.

Set mode of this layer to "multiply".

Filters->Render->Pattern->Jigsaw.

Set number of pieces wide, number high. Set Bevel Width to 0.

Save the "complete" puzzle.

Select the magic select tool

For each piece:

Select the puzzle layer.

Select a puzzle piece.

Select the background layer.

Cut the selection from the Background layer.

Save the image.