Increasing header height

[sharpiefan]

(Ironic, I know, given the previous post!!)

I'm using Practicality for my personal journal, and have inserted a header banner using this code:

#canvas {
background-image: url(http://i26.photobucket.com/albums/c139/Sharpiefan/Sharpebanner1.jpg);
background-repeat: no-repeat;
background-position: top center;
padding-top: 250px;
}

I want to introduce a small margin (of about 10px) at the top and bottom of my banner, to give a bit of space between the banner and the nav bar, at the top, and the header box (or whatever it's called) underneath.

How can I do this? Or can't I?

Trying to increase the padding number to 270 just moves the header box down, without shifting the banner itself down as well.