The Shale Style Folio has an overlay which can make finding the header image edit button a bit more challenging.

There is a small section in the upper right hand corner that you can select to edit the header. 

If you can't find the quick corner - you can always change the style to Riverstone or Onyx and change back to Shale.

Here is a quick video on how to edit this element in Shale Style Folios.