Can I see how works this plugin before buy ?
How can I add images to the content by the NextGen Editor? Is there a list of images to select one or I have to add URL?
Next question: is there a list of pages to add links inside of content ?
@tomasz_s1, There is a dedicated Github repo for all your questions about Premium products. There you will get dedicated attention from the devs themselves…
I can’t find answers for my question 
Is somewhere a demo of this plugin to see how it works ?
@tomasz_s1, If your question has not been asked before, you can ask your own questions in the Issues tab in the repo.
By the way, it is a good practise to create a new issue for each question your have.
Demo: I’m not aware of any demo environment. Would be nice though…