The default stock were saved out for vbulletins background for their software which is the reason why anything like a black or a dark background will have white jaggy edging showing.
You could request from someone (that deals with modifying the stock buttons) to correctly save the buttons set to your own matte background which would eliminate the white edging. It's a simple process of using the
Save for web feature in photoshop.
edit: Just noticed this is in the wrong area, You should request for this thread to be moved in the graphic discussions area.