In case you need instructions from the very beginning - here on dreamwidth you would go to http://www.dreamwidth.org/customize/options.bml and select "Custom CSS" under customize your theme to get to the text-box where you add in the code.
When I added a header image, I just did this (which is basically the second option in the link facetofcathy provided.
the background-position (I used right, because I had an image that "cut off" at the right side, and I wanted it pinned to the right of the display) accepts center, right, left, top, bottom, etc.
height is the height of your image, assuming you want it to fill the header from top to bottom. If you wanted the image centered in a sea of whatever your header color is, make the height larger than your image height.
I've probably left something out...
(ETA - I win at referring to code I didn't include in the snippet. sorry!)
no subject
When I added a header image, I just did this (which is basically the second option in the link
#header
{
background-image: url(http://www.example.com/image.jpg);
background-repeat: no-repeat;
background-position: bottom right;
height: 150px;
}
the background-position (I used right, because I had an image that "cut off" at the right side, and I wanted it pinned to the right of the display) accepts center, right, left, top, bottom, etc.
height is the height of your image, assuming you want it to fill the header from top to bottom. If you wanted the image centered in a sea of whatever your header color is, make the height larger than your image height.
I've probably left something out...
(ETA - I win at referring to code I didn't include in the snippet. sorry!)