Gap between header image and content

Hi iam a beginner to wordpress.iam using wordpress 3.5.1 twenty twelve theme. i have a big gap between the header image and the body content.i want to get rid of the gap and just leave posibbly a cm gap only.
my website is https://www.perthcityplastering.com.au.
anyone knows how to solve this problem? thanks for any reply and help