By default, the Lagoon theme uses the Default header layout (et_header_style_left), in which the logo is floated to the left, menu items are hard-centered, and a CTA button is floated right (if activated).
When the default layout is used, do not add more than 8 top-level menu items to the top-navigation. Doing so will overflow the menu beyond the header bounds.
Please note that the CTA button is available only when the default menu layout is selected. Any other type will disable its display.
The DiviBest PowerPanel Header options, included in Lagoon, gives you a variety of controls over the functionality of the header, beyond what the basic Divi theme provides, and you can easily use it to add a CTA button, define page-specific logos, different logos on scroll, hiding the header and more
Changing the Blog Header Background Image
To change the background image of the blog header, replace the blog-header.jpg file located in wp-content > themes > lagoon > images