First off I'm going to suggest having a look at Jigsaws Galore. You can download a free trial of the software. Jigsaws Galore comes with some jigsaws that you can start using straight away or you can use a picture file of your own and the software will convert it into a jigsaw. I particularly like this feature as you can make the jigsaw more interesting for the person using it by reflecting their interests. You can make the jigsaw easy or difficult by setting how many pieces the jigsaw will have, this is good as you can start the person off on a really easy jigsaw, then as they get the hang of it you can increase the difficulty.
Whats the advantage of this computer game over the traditional jigsaw? The person doing the puzzle on the computer is learning how to click and drag items on the desktop of a PC. This skill is essential for using a computer. Also some people on the autistic spectrum find it easier to communicate via a third party, in this case the computer is the third party. People love to show others what they have done (show and tell) and this can be true for people on the autistic spectrum too.