different logo on mobile layout

Hi there,

I want to have a different logo on the mobile version of the website I’m building. I tried the suggested code in functions.php but that doesn’t seem to change anything.

add_filter('avf_logo','av_change_logo');
function av_change_logo($logo)
{
    if(wp_is_mobile() )
    {
    $logo = "http://kriesi.at/wp-content/themes/kriesi/images/logo.png";
    }
    return $logo;
}

I’m currently developing the site on a local XAMPP, could that be a problem?

I also tried this code to add to my quick CSS as a test but doesn’t work either:

ul#mobile-advanced {
background-image: url(“http://d4e7wxbvl20c1.cloudfront.net/images.printconcept.com/blog/Tips_logo_design/logo1.jpg”) !important;
background-repeat: no-repeat;
background-position: right top;
}

What could be the problem?