and another thing, I would like to add some websites, how do you do that?
I haven't written proper documentation yet but you should be able to get something working without it.
Before you write code you need to learn how the website you want to add's API works. If it's a booru then they are all fairly similar. All boorus have a wiki/help section with information on how their API works.
Inside Zenith Grabber's "resources\website_handlers" folder contains each website's module. They're all pretty similar, just duplicate one (Danbooru might be the best to start from).
Firstly, two classes are created: UriOptions and WebsiteHandler; you can find some small documentation comments about them in the "resources\modules" folder to understand what they're doing. (Note: you don't need to supply the "unwanted_tags" part of UriOptions)
The only other thing in the file is the GetPosts function which is what turns the JSON or XML web response into an array of Posts. This part is quite simple long as you understand how the API you're accessing works. Make a request to the API in your browser and see how the JSON/XML is formatted so you know what to write here.