Well, good question. Sometimes it is possible and simple to provide such a reference picture. However, I generally avoid giving a print of the complete image because of issues of copyright.
Sometimes you will have your own picture for reference, e.g., if it is a photograph that you have a copy of; if it is a print from a calendar there is usually a small copy of the picture also on the back of the calendar.
If it is made from a print that you bought online, you can use the online image privately to help you. Or there may be other ways of obtaining images online of your print, just try using search engines.
If you yourself take a picture of the print personally for your own private use, I don’t believe that will be breaking the spirit of copyright laws. After all, if you were not going to have a puzzle made from the print, would you have bought it in the first place? I am not a lawyer, and some areas of law may seem grey, but I believe this is a reasonable interpretation.
I will, however, under normal circumstances send you an internet link to an image of your finished puzzle before you pay me the final amount, so that you know that it actually exists!
One last thing... If you do not have a reference picture for your puzzle, how much more interesting and challenging it makes it!
The material I mainly use at present is 4mm thick, 3-ply waterproof birch plywood. In its original state it is quite flexible (in one direction), but once a print is stuck to it and it is cut, it becomes rigid. It is a tough and durable material for the job - The British Jigsaw Puzzle Library uses it, and its puzzles are in constant use. It is light in colour, and has a reasonably solid feel to it.
And I have available 4mm thick 3-ply beech plywood (beech/spruce/beech). This is rigid and tough. This has got a lovely solid feel to it, and is a light shade of brown with a decorative dotted/stripey pattern of darker markings. The lighter coloured spruce making the filling for the "sandwich" contrasts with the brown, adding to the decorative effect, making a pleasing stripe around all the pieces. This is my present personal favourite.
I also have available 5mm thick, 5-ply poplar plywood. This is light in colour and quite light in weight.
The birch is the lowest cost, oak is higher, and the poplar just a little higher still. However, there isn't much difference in price. Your choice ought to simply be out of preference, because the difference in cost between them has negligible impact on the final price.
Other than that I use just the prints and PVA wood glue, water, and I may apply beeswax to the back of the puzzle if it seems appropriate. If you specifically do not want me to use BEESWAX, for any reason, please do let me know.
I deliver to anywhere that has a Postal Service!
It may seem surprising, but it is not considered necessary to add any extra protection on top of the print. All that is necessary is a good quality professional print, and this is quite adequate for the job. Additional layers, including laminates of plastic, also have a habit of coming off and becoming messy. For comparison, The British Jigsaw Puzzle Library does not use any extra protection on its some 4,000 wooden jigsaw puzzles.
For example, once I have the print and provided the wood for it is not special order, it will take 1 to 3 days to make a 500-piece puzzle, according to its complexity.