I saw a few closed topics asking about the image header for ocean mist, so thought I'd paste in the custom css that worked for me, in case anybody wants it.
Note that you need to upload your image to a page on your website, even if that page isn't showing. It just puts the image "up there" to be referred to.
For the #mainpic, set the height to whatever it is for your image. Mine was 114px. Note that the tabs will overlap it.
For the #header, you'll need to mess around with the min-height to get it just right.
background: url("your image URL here") no-repeat;