There is a really neat video editing software that I use and you can get it for free without piracy or buying it. Is called Camtasia, And it also can record your PC screen as well. And for thumbnails? I use paint tool sai, Is also free to use.
Paint.net, Gimp, Pixlr as far as editing I have no idea. There are people who say you can get a version of Photoshop for free and legally I don't know if its true but you can look into the as well.
Photoshop CS2 photo editing for free legally and Adobe Premier Pro CS2 for video editing, also I have a video on that if you do not know how and where to get it.