Acrobat custom stamp adds white background to PNG signature with transparent background

Dear all,

I have some signature files in PNG format with transparent backgrounds that I often use to sign Word or PDF documents. For PDFs, I usually add the signature as a stamp, and everything works great. I just tried adding two more signatures to my palette, and for some reason Acrobat is adding a white background and a huge white border around the stamp (even though the original PNG file is cropped exactly around the signature with no extra space). So I'm having to make the signature really small to fit it inside this signature field. I've tried just pasting the image instead of using the stamp function, but when I do it that way, the file looks really crappy. Any ideas? 

Thank you!