[Plugin: NextGEN Gallery] Large Space above gallery on website

I have installed the Nexgen Widget in the homepage Ads and included some images in the gallery. There is a large space above the adverts and below the top navigation menu. How do I reduce the size of the ‘gap’. You can see it here https://www.top100childrensbooks.com/ I have downloaded firebug. Which style sheet should I change as there seem to be so many? I am a ‘newbie’ at this so all seems very confusing! Many thanks.