Complex sentence: When the puzzle is nearly done, the piece that was missing the whole time is found under the table at the boy's feet--in fact, that piece completes the picture, and the portrait of the little boy in the puzzle is finally complete.
Split into simpler sentences: When the puzzle is nearly done, the piece that was missing the whole time is found under the table at the boy's feet. When it is put in place, the portrait of the little boy in the puzzle is finally complete.