Code for adding a picture/logo on the left side of header.

I take it that you want the logo to appear to the left of the blog title. Is that right?

Basically, you have two ways of doing it. You can use a background image for the <header> element, or you can add a foreground image into that same element, probably in the header.php file just before the site title <h1> element.

Either way, you should consider doing it in a Child Theme. Child themes are the only recommended way of modifying themes.

Obviously, once you have added your background or foreground image you will have to modify the (child) theme’s CSS to position it exactly how you want it. How you do that will depend on which strategy you adopt.

HTH

PAE