How can you make an image fluid to be part of a fluid layout in Adobe Dreamweaver CS6?

I am trying to make the header image fluid. I made a header div tag and I put the image into the header, but when I preview it in the browser the image is fixed but the header is fluid. I made a new ID selector for the image and changed the Box category to 100% width and 200 px height and I also put the float and clip to the left but it did not change anything.