From one of your comments:
I want somebody to create 3D photorealistic images of the product,
before I start application for the patent; if the guy steals my idea
or passes it to his friends/relatives (who might be able to
commercialize the idea faster than me) there will be no impact on his
reputation. What should I do in such a case? Will I be able to prove
later on during patent filing that I was the real author of the idea?
How can I prove that I and not his friend X was the author of the
You really should consider filing a provisional patent application. These are generally easier to file and will provide one year of protection to allow you to draft and apply for a non-provisional application. This will provide the proof you need to show you are the inventor.
That said, you could consider contracting with an actual product development firm. They would have a lot to lose if they violated confidentiality. This would probably be somewhat more expensive than a solo contractor but I've never heard of a product development firm stealing inventions, ever.