User blog comment:Kassyndra/Ling Zu, the Petal of the Dusk/@comment-24132411-20140311172306/@comment-6292556-20140312170720

Well, it has to be the actual name of the file, not just the mastery's name. My Lunar Song's icon is not just 'Runic_Blessing.png', it's 'Runic_Blessing_mastery_s4.png'.

To find the file name, you can open image in the new tab and look at the end of the url.