Change format of header image

Author: John Yorke

_config.yml

@@ -54,7 +54,7 @@ topbar_color                              : "#000000"
 topbar_transparency                       : 1.0
 topbar_title_color                        : "#ffffff"
 
-cover_image                               : assets/headerimage.png                    # Replace with alternative image path or image URL.
+cover_image                               : assets/headerimage.jpeg                    # Replace with alternative image path or image URL.
 cover_overlay_color                       : "#000000"
 cover_overlay_transparency                : 0.5