Greyzed is a little of a challenge because the of the overall background image that is in #wrapper. You can see it here: http://s0.wp.com/wp-content/themes/pub/greyzed/images/bg.jpg . As you notice, there is a blank area where the sidebar and content are, so stuff can't be shifted around very much to make room for a header image.
You could incorporate the site title into an image and then put the image in the area of the site title. You have about 635px x 180px of space there for a header image (where it doesn't overlap the arced arrow for the RSS feed link.
For a full-width image, you are limited to about 1035px x 100px, and then the site title would have to be below that image.