Flato header: How can I upload an image

I want to upload a photo logo to my Flato theme and I want it to cover the whole width of the banner. Like it now does on the .com version of my website (https://ontbijtjeopzaterdag.wordpress.com).

But I can’t get the .org to look exactly the same as the .com. When I upload the logo it is too large and overwrites the blog title.

Is there a way to solve this problem? I’m working on a MAC and have no knowledge of CSS….

Thanks in advance!