I've solved my problem this way:
1. disable both thickbox and lightbox modules
2. try one by one all codes proposed by people in this thread.
3. only the code by matshep1 worked for me.
4. enable thickbox module and...yahhoo!!! It works!
Thank you all and matshep1 especially!
2 matshep1: if you mean that you need to show all images of a product in thickbox window (switching by 'next' button) then you should look to the thickbox module settings. There is an 'Image field gallery' option. If that's not a problem you mentioned sorry my stupidness