Transparent header image

Is it possible to have my header image be transparent? I have crated an image in photo shop with the back ground removed, but when I add it as my header it seems to fill in the back ground white. I have tried it as a .PNG file type. I have been looking through the style sheet and I don't see where the back ground is being added. Is the theme auto adding a back ground or am I doing something wrong with my picture? If it is in the code can you show me what to part to change? Great theme, it is better than any of the purchased ones I have used in the past.   Thanks for any help! My site: http://www.gettingstamped.com