Insert larger header image / logo

I’m using ShopperPress v7.1.4 with the default template and blue style. Does anyone know how to change the background header area just above the menu bar? I would like to add a single large graphic to this area that will basically be the logo. I’ve tried adding a grahic that inserts itself across the whole width of the page just below the social media icons but I can’t center it or place it below the icons.

Any suggestions welcome.