How to place custom icons (not emblems) on folders in Thunar

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http://askubuntu.com – (This is not a duplicate as this question describes how to add custom icons to folders in Nautilus, whereas there seems to be no explanation for Thunar.) In Nautilus (as this questions shows) there is the fun feature of being able to place any custom icon onto a folder by just right clicking the folder, clicking the folder in the menu that comes up and selecting a custom icon. However in Thunar I can only seem to add emblems (see the screenshot below) and not actual custom icons (I have a folder of them scaled to the right sizes ready to use). I have looked round the Thunar settings and gen (HowTos)