Adding an image to the header in RoboHelp 2017

I am trying to add an image to the header in RoboHelp 2017, but have had no luck so far following the threads on the forum. I managed to update the "Home" image at the left of the header, but unfortunately I could not add a large enough image. I understand that I probably need to edit layout.css, but would really appreciate some advice on doing this. I am outputting to Responsive HTML5, using the Charcoal_Grey layout.

Thanks,

Mark