Can you try adding it?

Note that the query string I added to the end of the image URL in the example above was a random size I picked because you never mentioned what size you wanted before. You can make it even smaller. Just change ?w=700 in the image URL to a smaller number.

At http://allthingsattainable.com/ the header image area is currently 990 x 150 pixels.

The image you linked is also 990 x 150 pixels: http://allthingsattainable.files.wordpress.com/2012/01/cropped-all-things21.jpg

If you just wanted to shorten up the space above the image, you could do this:

#branding {
  padding-top: 0;
}

And if you wanted to make header image smaller, you can adjust the hgroup background image like this:

#branding hgroup {
  background: url(http://allthingsattainable.files.wordpress.com/2012/01/cropped-all-things21.jpg?w=700) no-repeat center;
}

Thanks!