verify linked content for css images

I am using verifyLinkedContent to verify page refs to css, js, images.
How can I verify with webtest that an image referenced from a css style is
also present?
For instance, the image reference in this style:

        div#header {
            color: black;
            clear: both;
            margin: 0;
            padding: 0;
            height: 112px;
            width: 100%;
            background: #fff url(../custom/banner.png) bottom left repeat-x;
            position: relative;
        }


With firebug, I can see some of these are requested but not served.

Cheers,
Josep