Adding background header image

Site URL: http://pug-tetra-xkyd.squarespace.com

I have been trying to figure out how to add a background image to the header site wide. (Global). I have failed every attempt. I found some help but not enough to successfully work. Would someone please direct me to the instructions or give me instructions.  As a noob I appreciate any help you can provide. 

It looks like you solved with this CSS?

.header-announcement-bar-wrapper {
    background-image: url('https://static1.squarespace.com/static/6064ea38523204155155146c/t/6068d752ce245a430c6d4d9f/1617483602909/wood_background_1024x678.jpg');
    background-repeat: no-repeat;
    background-size: cover;
    background-position: center
}