Move header image to left side of header and resize header image

I’m currently using the 2014 theme and latest version of W.P.

I’d like to take the header image, which currently sits above the header, and move it inside the header to the left side, opposite of the menu. And I need to resize the image to fit inside the header.

I’ve set up a child theme with a style.css sheet. But after lots of reading and research, I can’t determine if I should customize it in the style.css or function.php. Please advise, I’d appreciate the help. Thank you