How to move up main content

Hi everyone.
I not a big expert of css and wordpress, but I’m trying to make my own website and need an help to solve this problem:

I’d like to move up the main content of all the pages.
For example , in the home page, I’d like to see the pictures not in that position, but in the center of the white blank area (so more up than now)
And this I’d like to see for every kind of page, even where there are just written posts (like “About page”).
My website is this and is almost empty because I’m doing some tests.
I’ve fixed the position of site-content in my stylesheet of my twenty-fifteen child theme, because if I don’t do this, a scroll bar appears to see the entire picture in the home page (and everywhere of course) and I don’t want it.
I’ve also modified the width because I needed a sidebar menu smaller and a bigger content area.

Sorry if I made some grammar mistakes, I’m not native english and I wish to had been clear for my problems.

Thank you.