THE VENDETTA

I want to cut a render of someone out and paste it in a separate picture, but when I do so the gray and white checkerboard is pasted with it....However when I use Gimp it does not do this...Please point me in the right direction as to how I can paste a render without the gray and white checkerboard background with it... note: i am trying to do this with paint.net and not gimp ~Vendetta