Graham GuruzLord it is just set as a background image from within the twig file it's available in the twig data arrary in set sizes up to 150px (16,32,64,128,150)
I use this tacked on to the twig to show the contents of the data array

{% if translate ('debug')=='1'%}<div class="panel panel-default"><div class="panel-heading">Debug Information</div><div class="panel-body"> _context twig global <textarea class="form-control" style="height:10em;" >{{ _context|json_encode(constant('JSON_PRETTY_PRINT')) }}</textarea> </div></div>{%endif%}

and then I set debug=1 on the forum translation file to toggle it