logo for desktop AND different logo mobile

Answer this riddle and make my dreams come true! Because then I can go to sleep finally…
I’ve been tinkering for hours and it cannot be this complicated.

What chunk of code would switch out the header logo url correlated to different view port sizes?

Client wants this one for desktop:
And this one for mobile:
—————————–

I found this suggestion to add to function.php, but it doesn’t seem to work in my case:

add_filter(‘avf_logo’,’av_change_logo’);
function av_change_logo($logo)
{
if(wp_is_mobile() )
{
$logo = “http://calrestrats.com/wp/wp-content/uploads/2016/02/CRS-text-01-01.png”;
}
return $logo;
}
————————

THANKS

Maybe in future versions of the theme there will be an option to set the logo for desktop vs. mobile since the layout changes from horizontal to vertical??