Unfortunately if you place it as an image, the page will have scroll bars and the only way to avoid that is to use a background which is not an option.
Personally speaking, you should really consider making the header image that can stretch to any screen size because "extra-wide" is a matter of opinion rather than a stated width. For instance, let's say I have an old laptop with an 800x600 resolution, a cheap Dell monitor with 1280x1024 resolution and a nice iMac with a 2560x1440 resolution. Would the extra wide only fit the iMac? Or maybe it would look ok on the Dell?
The point I am trying to get at is that first, sit down with your client and go over their web stats to determine an optimal width, not just extra-wide because it looks nice. Then cut up that header and code it in CSS with repeating backgrounds as needed and static images on top.