Header Image presents differently under 4.0

[depelton]

RE: My brand new site at http://sierravoices.com/

Novice question: I'm running WP 2.8 and since I upgraded from Lifestyle 3.0 to 4.0, the code I modified in style.css to "stretch" the header.png image to the right no longer works. The code I modified looks like this:

background: #85ACAC url(images/header.png) no-repeat;

... and

.headerleft {
width: 960px;
float: left;
font-size: 14px;
margin: 0px;
padding: 0px;
}

.headerleft a img {
border: none;
margin: 0px;
padding: 0px;
width:960px;
}

.headerright {
width: 0px;
float: right;
margin: 0px;
padding: 20px 0px 0px 0px;
}

This worked (stretched the image all the way to the rightmost border) under 3.0 but not under 4.0