If someone account is banned/deleted all uploads of this person are deleted?
I'm don't want to be afraid that in the future my favourites get deleted.

Nope, the images stays untouched (Unless they break any of the rules)

Posts are only deleted if they break site rules.

There is no way to delete a user's account.

I mean, as long as you don't have anything stupid in mind planned for the future, I wouldn't get worried about being banned to begin with.
This rule is valid both for you and any other user on this site, including the uploaders of your favourite pics.

Plus, as Alkin and Jerl already said, as long as an image doesn't break any rules, it stays regardless of its uploader being banned or not, so.

This site have some "rare" images AKA the original source is long gone so the way is to get it from here, I am glad to know that even if someone is banned the images remain intact.

gelbooru is a really lax site. just use it for a few months and then things will make sense