Fix error if bgimg param is not added to config

layouts/partials/head.html

@@ -33,7 +33,7 @@
     {{ printf `<link rel="%s" type="%s" href="%s" title="%s" />` .Rel .MediaType.Type .Permalink $.Site.Title | safeHTML }}
   {{ end -}}
   {{ template "_internal/google_analytics.html" . }}
-  {{ with resources.Get .Site.Params.bgimg -}}
+  {{ with resources.GetMatch .Site.Params.bgimg -}}
   <style>
     body {
       background: #ecedef url("{{ .Permalink | safeCSS }}") repeat;