how do I get rid of space between nav bar and background image?

Background pic

How do I get rid of the gap between the nav bar and the background image. On top is the link to the screen shot of the gap.

Here is my css code

#header{
background-image: url("sizepic.jpg")
width: 100%;
height: 100%;
  background-repeat: no-repeat;
background-size: cover;
}

Thank you in advance for your help.