I have added a header image to a page so that the slider style banner remains through-out the site (however without the slider capability on the child pages). When I upload the image to the page as media it pads the image so that there is white space around it.
So, you want to remove space around header image. If not, please make us clear.
We need to inspect your site to give you a solution. Can you send us a screenshot of the page with inspect element open at that area?