I put in a partially transparent PNG image in Tkinter and all I get is this

How do I make the dark triangle on the right clear? (like it's supposed to be)
This is python 2.6 on Windows 7, btw.
I put in a partially transparent PNG image in Tkinter and all I get is this

How do I make the dark triangle on the right clear? (like it's supposed to be)
This is python 2.6 on Windows 7, btw.