add image to theme dir and use for fallback

layouts/partials/page-header.html

-{{ if .Params.featured_image }}
-  <header class="cover bg-center" style="background-image: url('{{ .Params.featured_image  | absURL }}');">
+{{ $featured_image := $.Param "featured_image" }}
+{{ if $featured_image }}
+  <header class="cover bg-center" style="background-image: url('{{ $featured_image | absURL }}');">
     <div class="pb3-m pb6-l bg-black-60">
       {{ partial "site-navigation.html" . }}
       <div class="tc-l pv6 ph3 ph4-ns">

layouts/partials/site-header.html

-{{ if .Params.featured_image }}
-  <header class="cover bg-center" style="background-image: url('{{ .Params.featured_image | absURL }}');">
+{{ $featured_image := $.Param "featured_image" }}
+{{ if $featured_image }}
+  <header class="cover bg-center" style="background-image: url('{{ $featured_image | absURL }}');">
     <div class="bg-black-60">
       {{ partial "site-navigation.html" .}}
       <div class="tc-l pv4 pv6-l ph3 ph4-ns">