Pasted signature turns black?

I created a written signature with a transparent background and saved it as a PNG. When I paste it into a word processor like TextEdit the signature appears black with a transparent background, but when I paste it into a PDF I'm creating it appears as black on black. Well, obviosly I'm not highly skilled at Acrobat, but I'm wondering why this is? What am I doing wrong?

I have Adobe Acrobat 9 Pro.