Can you edit everything?

I’m just trying out a website on here to see if it’s worth paying for an account to move my blog to. But it doesn’t look like I can actually edit the pages completely. I want to move things around and change the width of the side menu etc, but there’s no way that I can do this. I found a design I like, but how do I edit more than just the text and pictures? And am I only limited to putting images where they already are? I’d like to put a large image at the top of the page, how do I do that?

Thanks 🙂