#hugeimage
{
    background-image:url(http://example.com/hugeimage.jpg);
}

ready.mobi appears to assume that every stylesheet image is used on the tested page. hugeimage.jpg is still included in the page size test even though the #hugeimage element doesn't appear on the site.

I understand the nuance of how and why this is done and wouldn't bring it up at all if the effect on the result weren't so disastrous; it single-handedly dropped my score from 4.8 to 3.1. It's also a bit of a pain for the developer to track down the problem, since the CSS doesn't apply to the page there's no reason to check it.