Generally, that's a better idea anyway. All the same, what I meant was that you should open the graphic in your editor (such as Photoshop) and mark the center point. Then measure 530px either side of that, and make sure you don't place the logo, or anything else, outside of those boundaries. That way, the logo will appear within the boundaries of your wrapper, which people will see even in narrow browsers.
But yes, separating the logo (and any text) from a background image is a good idea. Ideally, the logo should be in the HTML, with fallback text for those without images on, and for search engines etc.