Using png files in hot image slider

Hi all- i was just wondering if anyone has tried using png files for the images? I am finding that it is adding a black line around the outside (almost as if it is rendering them as gifs).

I need to use png as the 'slideshow' is in the header and is an unusual shape...

If anyone can give me any help on how to fix this it would be much appreciated. Site has not been delegated yet but i can give the ip details to anyone who is willing to assist.

I look forward to hearing form someone! thanks

Kind regards, alicia