Making a Slideshow/ Widget under Header not inside Left Sidebar

Hello, I am trying to put a slideshow under the header, but I would live it to not go into the left sidebar. Basically I would like it to start right when the white space of the page starts and not go into the black area. Any idea on how I could do that.

Also I was wondering what is using in the display picture of the theme that shows thumbnails of posts under the header.

Thank you