So I messed a bit with the engine, until I got:
* A full screen canvas element (which KineticJS calls Stage)
* An external image loaded (featuring Princess Merida)
* Such image being splitted into rectangular pieces (currently I am working on “real jigsaw” shapes)
* Every piece ca be dragged and dropped around the canvas
* Picked up pieces are in front of the rest of the puzzle
This is the code, still unexplained as I am still experimenting with it
And you can see the result at this page.
The result is interesting in my opinion, at least with only a few lines of code, but I found it a bit laggy on mobile devices.
Anyway, stay tuned for a real jigsaw puzzle.